My 3 yr old son went to the playground and returned with a toy car he found in the sandbox.
It was definitely an aveidah because there was nobody else in the park at the time. It is a park mostly frequented by Jews.
1) Are we required to announce it?
2) How does one go about announcing it practically?
3) My son was playing with it the whole day, should I have taken it away from him since it really belongs to whoever lost it.
1) If the toy has identifying marks, or has a unique feature to it, you should announce it by placing a sign in the vicinity of the playground. If there are no identifying marks there is no obligation to do so, but if the toy is valuable it remains correct to make an effort to return it to its owner.
2) As noted, by placing a sign in the vicinity of the park, or other place where local people will see it.
3) Insofar as there is an obligation to return the toy to its owner the toy should not be used. However, if there is no clear way of identifying the toy and therefore no obligation to return it, it can be played with.