S.H.
How old are the kids?
If they don't yet write... perhaps you can, write a poem, that they dictate to you (it doesn't have to be all rhyming or professional), but just about their Dad and how they feel or like about him or appreciate him for or how they make them feel, happy etc.
Then, either hand write it on paper or print it out from the computer, and frame it.
It will be original, one of a kind, and from their heart.
Priceless.
all the best,
Susan