I don't see why sending them seperately should be illegal.
Kinder Surprise chocolate eggs are illegal in the USA under 21 U.S. Code § 342 - Adulterated food (emphasis mine):
A food shall be deemed to be adulterated—
(d) Confectionery containing alcohol or nonnutritive substance
If it is confectionery, and—
(1) has partially or completely imbedded therein
any nonnutritive object, except that this subparagraph shall not apply
in the case of any nonnutritive object if, in the judgment of the
Secretary as provided by regulations, such object is of practical
functional value to the confectionery product and would not render the
product injurious or hazardous to health;
combined with 21 U.S. Code § 331 - Prohibited acts:
The following acts and the causing thereof are prohibited:
(a) The introduction or delivery for introduction into interstate
commerce of any food, drug, device, tobacco product, or cosmetic that
is adulterated or misbranded.
So the eggs are only illegal because the toys are "imbedded therein". If you take them out, I cannot see any reason to object to them.
To be on the safe side, you might want to remove (and not send along) the original packaging of the eggs, maybe even ship the chocolate egg opened. Otherwise customs may recognize the eggs and confiscate them without realizing you removed the toys inside.