While it may appear "creepy" on the surface, perhaps he's socially awkward or just doesn't have anything else to do and the children remind him of an earlier time in his life which he yearns for again.
Instead of considering whether what he's doing is illegal, why not consider treating him like a neighbor and offer him some neighborly advice that his staring is freaking your kids out. Or invite him over sometime so you can secretly "interview" him to see if he's actually creepy or just an old man who is watching kids play. You may discover more about him that will either confirm your fears or eliminate them.
If you conclude he really is doing or planning something nefarious, then you'll have a better idea of what it might be or you may then be able to warn him that you're watching him watch your children.
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Creepy, but not illegal as long as he stays in his own yard and does nothing more than make awkward comments.
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