Need Fast Working and Child-Safe Dog Deterrent

Squirt him a bit with the hose when he goes somewhere you don't want him to,or just use a spray bottle with water .
This is natural behavior on his part. The old place is thoroughly marked now ,so he needs a new place.
Of course if you could possibly arrange for a different dog to mark the old place while he was elsewhere,then he'd have to mark it all over again to make it His Again.
& this may be what has happened at the new place. Maybe he smells the children's dogs from whatever scent their clothing left on the equipment.
& cover the sandbox with something if you have a sandbox.

My dogs had a problem with going to the bathroom on the back porch instead of in the back yard so I sprayed down the back porch with vinegar. Just fill a spray can with plain white vinegar and mist away. Dogs refuse to go to the bathroom where there are vinegar smells/fumes. It works wonders and the vinegar smell sticks around for at least the entire day. I also spray vinegar around "problem areas" in the house where the dogs are prone to have accidents. Something about the acidity of the vinegar reminding them of the ammonia in their urine prevents them from going to the bathroom there. I've also known people who have completely house trained their dogs by using just vinegar. They do this by putting a vinegar soaked cloth up to their dogs' noses if they're caught going to the bathroom in the house. Hope this helps!

How about a fence! Or, take him to his old spot on a leash for several days, praise and treat him when he goes there. Also, be sure the play area is picked up and washed so he isn't tempted to go back. Vinegar is kid safe and should help to eliminate the odor that keeps him coming back.