C.E.
A DVD is great -another idea might be a picture collage - you can do it yourself with the girls and even frame it. They can write a note to go along with it.
If they are old enough, you could start now and have them do a little journal for their Moms. They could write about memories they have, things they've done together, feelings they have for their Mom....
Another thing my son did one year for Mom's day was to paint a flower pot for me and he gave me a plant. Take a plain flower pot and let the girls paint it - any design they'd like, maybe even paint a note on it, and then buy a plant to put in it and give their Mom.
I think it's great that you have such a good relationship with all of them!
God Bless-
C.