Thank you for your query.
Please ask your child whether someone is kicking him on his ankles or not not.Ask him whether he had a bad fall or some injury
to his ankles.
A bad bruise
might be associated with small dot like hemorrhages nearby.
If the joints are swollen and there is a habit of easy bruising in your child then he needs investigations.
If these are not there apply a balm for the pain and give him some paracetamol
Hope this answers your query.Wishing your child a speedy recovery.Have a nice day.