I would serve the parents (certified mail), with a "cease and desist" letter, telling them that the children are repeatedly trespassing on your property and that you want them to stop; even get the police involved if you have to. I know it sounds harsh, but you said New England; that's where I live and I know the trespass laws are not in your favor ... especially when it comes to kids. Take Connecticut as an example: This is their law on trespass and kids (not just attractive nuisance!):
A possessor of land owes each person who enters his land a certain
duty of care based on the person's status. The legal significance is
that a possessor of land has the duty to an invitee to inspect the
premises for hidden defects and to repair or erect safeguards, if
necessary, to make the premises reasonably safe. He has no duty to
inspect or to repair or erect safeguards for licensees. But he is
liable if he knows of a condition, realizes it involves unreasonable
risk, has reason to believe the licensee will not discover it, and he
permits the licensee to enter or remain without warning or making the
condition reasonably safe.
Generally, an owner owes trespassers no duty of care because he has no reason to expect them to be on his property. Therefore, he does not have to warn or protect them from potentially harmful conditions on the property.
However, an exception applies if a property owner knows, or has reason to anticipate, that children will trespass on his land. In this case, a special duty arises and the owner must take steps to protect children from any of the property's dangerous conditions. The post you just made indicates even you think that the rock walls, or other "normal garden features" could be dangerous; and they can be!
The law requires that you take reasonable steps to eliminate the condition or by otherwise keeping children away from it.
DUTY OWED TO TRESPASSER
In Connecticut, the following rules apply to a possessor of land with respect to a trespasser.
- He may not intentionally harm the trespasser or lay a trap for him.
- The trespasser is entitled to due care after his presence is actually known.
- There is no duty owed regarding the condition of the premises.
- The possessor of land has no duty to trespassers if he is engaged in a dangerous activity until the person's presence is know.
- The possessor of land has no duty to warn trespassers of dangerous hidden conditions (Conn. Law of Torts, § 47).
Duty Owed to Trespassing Children
Connecticut's appellate courts have adopted the Restatement (Second) of Torts rule regarding the duty of a property owner to trespassing children (Duggan v. Esposito, 178 Conn. 156 (1979), Neal v. Shiels, Inc., 166 Conn. 3 (1974), Greene v. DiFazio, 148 Conn. 419 (1961), Wolfe v. Rehbein, 123 Conn. 110 (1937), Yeske v. Avon Old Farms School, Inc., 1 Conn. App. 195 (1984)).
Under this rule, if an owner knows or has reason to know that children
will be on his property, he has the duty to protect them from injury
by either fixing the harmful condition or ensuring that the children
will not have access to that part of the property.
The rule states that a possessor of land is liable for harm to
trespassing children caused by an artificial condition on the land if
(1) the possessor knows or has reason to know that children are likely
to trespass in that place, (2) the condition is one the possessor
knows or has reason to know and should realize will involve an
unreasonable risk of death or serious bodily harm to children, (3) the
children because of their youth do not discover the condition or
realize the risk, (4) the utility of maintaining the condition and the
burden of eliminating the danger are slight compared with the risk to
children involved, and (5) the possessor fails to exercise reasonable
care to eliminate the danger or otherwise protect children
(Restatement (Second), 2 Torts 339).
Put in the letter that you are disclaiming any liability for injury to them that may occur on your property, and make them aware of all the ways they could be injured – so they've been informed. You don't have a duty to remove rock walls because unsupervised kids jump off them. They are not invitees, they are trespassers. So make it known you do not want them on the land and for any further breach you will call the police. Because otherwise you could be responsible.
Using CT again as an example, you could include the legal statute about trespass in your notice:
Trespass Crimes and Infractions
A person commits first degree criminal trespass when (1) he enters or
remains in a building or any other premises after the owner or an
authorized person personally communicates an order to leave or not
enter and (2) he knows that he is not licensed or privileged to be
there. This crime also applies to entering or remaining at a place in
violation of a retraining or protective order. This is a class A
misdemeanor punishable by up to one year in prison, a fine of up to
$2,000, or both (CGS § 53a-107). A person commits second degree
criminal trespass when he enters or remains in a building knowing that
he is not licensed or privileged to do so. This is a class B
misdemeanor punishable by up to six months in prison, a fine of up to
$1,000, or both (CGS § 53a-108).
A person commits third degree criminal trespass when, knowing he is
not licensed or privileged to do so, he enters or remains in any
premises for hunting, trapping, or fishing or enters or remains in
premises that are posted in a manner prescribed by law or reasonably
likely to come to the attention of intruders or that are fenced or
enclosed to exclude intruders. This also applies to state lands near
state institutions. This is a class C misdemeanor punishable by up to
three months in prison, a fine of up to $500, or both (CGS § 53a-109).
It is a defense to these crimes if (1) the building was abandoned, (2)
the premises at the time of entry were open to the public and the
person complied with all lawful conditions on access and remaining on
the premises, or (3) the person reasonably believed that the owner (or
someone else with the power to do so) would have or did license him to
enter or remain on the premises (CGS § 53a-110).
A person commits simple trespass if, knowing he is not licensed or
privileged to do so, he enters premises without intent to harm any
property. This is an infraction punishable by a fine, currently $77
plus costs and fees if paid by mail (CGS § 53a-110a). A separate
infraction covers trespass on railroad property when a person enters
or remains on the property without lawful authority or consent of the
railroad carrier. This is currently a $121 fine plus costs and fees if
paid by mail (CGS § 53a-110d).
You could just substitute your state's laws if you're in MA, or RI, or wherever. You could have a lawyer draft this letter for probably $200 (free if you have a friend who practices :~) and that will really scare them.
Tell them they will be liable for any damage the kids cause/or may cause to your property. But without doubt, put them on notice!