Yes and no.
There are numerous cases where criminals, upon breaking in to somewhere, find evidence of a worse crime and notify authorities. This will provide reasonable suspicion enough for entering the scene. Generally, in testimony, Statements against Interest are more believable because a burgler wouldn't admit to breaking and entering if he had a way to explain why he was there in the first place.
(Example: Alice breaks into a Warehouse and sees a mutilated body and blood everywhere. Alice immediately stops her theiving ways and calls 911 to let them know about the scene. Whether or not Alice stays, a dead body is enough probable cause to secure the crime scene without warrant. Its in Alice's interests to stay and help as there is trace of her at the scene and she would be pegged as a suspected murderer. If she's picked up and admits to calling the cops, it's good, but staying and helping out after the call will likely get her off on the charges related to the murder.).
It could also work if they are persuing one crime and discover evidence of a second unrelated crime.
(i.e. Alice robs the factory and gets away. The Factory Foreman calls the cops to investigate the crime scene, which at this point, does not need a warrant. While investigating, the Cops find security footage that Bob, the night guard, killed Chuck, a late night worker, removed his body, and cleaned the scene, all before Alice broke into the factory. The outcome of the case being made against Alice does not affect their need to prosecute Bob, as they obtained that evidence while looking for Alice in a valid investigation, not Bob, thus it is legal).
Under these situations a crime that leads to a separate valid crime involving a different party is admissible. There are two possible reasons that the attorney might think this:
Fruit of the Poisonous Tree: This is the obvious element... the kids committed a crime with the hopes that the cops would use the evidence found by them in their commission of a crime to get the real bad guys. The attorney parent thinks this is stupid because the kids broke in specifically to do this and thus any evidence is now tossed out of court. This isn't usually the case in how this scenario will play. Generally the cops are more than happy to look at evidence obtained by criminals that points to another crime. In fact, this is how a lot of gang enforcement units and drug enforcement units operate... pick up a small fish and cut a deal for evidence against a bigger fish (turning state's in the criminal lingo, as the witness is becoming State's Evidence to another crime). As long as it's given to the cops as part of legitament evidence seeking, the cops can follow the leads where ever they... er... lead...
Chain of Custody: This is probably, if properly thinking, what the attorney parent is thinking that's a bit more probable. Lets say these kids found a dead body with a sword in it and take the sword to the police... this could get dicey as the kids have contaminated the evidence in possible ways that the killer's lawyer could get thrown out. One thing CSI doesn't always show (though there are a few episodes where it comes up, but not many) is that when something is taken in as evidence, it is carefully documented, sealed, and tagged with a check in/check out list. Every time the seal is broken, the person breaking the seal notes the time, date, and reason and when does, reseals it with a new seal, and signs the time and date of the seal again. This is so at trial, the attorneys know exactly who opened up the evidence, what they did, and what possible contaminants were introduced. You even have to sign into a crime scene before you go up to the yellow tape. A good defense lawyer would call into question any evidence from anything the kids handled to get the evidence tossed (i.e. Your honor, these Meddling Kids handled the sword without following the chain of evidence. They even let their dog handle it. They had already harrassed my client earlier today by insunuating that he was involved with a hoaxed paranormal activity to scare people away from the factory. Since they claim they found the sword, but did document it at the scene, we don't know anything about it prior to the police's chain of custody. I motion that the evidence be dismissed.)
If this is successful, anything from the sword is now no longer admissible as if the sword had never been found (including blood of the victim on the blade and finger prints of the suspect on the hilt)... in effect the evidence was prossessed as best the police could but the veracity of the story of it's discovery is too questionable to be considered. The defense does not have to be right, he just has to show there could be another explanation for the sword and the evidence linking his client to the crime committed by it.
In short, without specific details, the attorney parent could be right or could be wrong, or more humorously, right, but for the wrong reasons.
Edit: U.S. only. See other answers for other jurisdictions.
