I.M.
I would give them a ring too. Just so they know that he is not only getting married to their mother but to them as well. So they see and understand that they are as important to him as they are to their mother. And if they can I would do a small inscription in the inside with the date of the wedding, names and a heart or something of the sort :)
Just remember that they don't have to wear it all the time, they can also put it on a chain or necklace and wear it that way :)
Hope this helps