I did this with my son ( was due the week before Easter) I just used craft paint. Let it dry, took pics, washed it off. Don't know how safe it is but I know we're both fine 2 years later.
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If I was to do this I would probably just rummage through DD's art supplies. They're all non toxic and by the way the paints stick on her when she paints in sure it'll work fine
Re: Safe Body Paint- Easter Egg Belly