You must feel so hurt.Explain to the children what a wo nderful fun day Mothers Day is,build it up,give them paper and crayons to make special pictures etc.Then if you can afford it go shopping for something you fancy and let them wrap it in something,see how you that goes ! Make waffles etc on the day,then try to do something you have always wanted to do ( probably with the children by the sound of it)
Start a tradition now,also I'd let MIL know, if possible,it's her bringing him up is the problem I guess you can't say that,pity.I had a similar problem in the past until the children always asked "What are we getting Mummy ?"
You are doing a very good job,thinking of you.