Thanks for writing in to Healthcare Magic!
I think they are due to allergic tendency or a bite by an insect say an ant.
When he comes after playing watch him again if the problem happen again it is allergy
and the child should wear proper thick dress and should not expose there by urination or any such need.
If there is no such thing in following days then may be it was a bite, so do not need to do anything further.
If you have any further query please let me know I would try to answer it for you.
Wish you a great health!