It's a fantastic idea, it really isn't the amount of money that 'counts' and such a gift is so unique that i'm sure your family members will love it.

I have actually been playing around with a similar idea myself but wondering how i could get both DS's prints on a canvas without messing up 10x before i was happy with it

Last Christmas i actually had paper and pencils out for DS (he was about 22 months at the time) and he drew as he pleased and i kept the drawings. I printed out a yearly calander off the computer, added a few small photos of him to the drawing, laminated, bound the two pages together and gave the calanders as a gift to grandparents & aunt & uncle. It's a keepsake that they will treasure forever.

Stick with your idea it's great