Oh, yes! Toddlers love to fill empty spaces. If it is empty, they will find it! Size or shape of empty space, doesn't matter. Just as long as it's empty! And if it doesn't have the possibility of filling up, ever, (like a cat door with a garage on the other side), all the better for a toddler. Think of the amount of stuff they can gather from around the house, pushing it through the hole, while someone (me) spends her time retrieving, because child wants his stuff back!
I'm here to serve,
P.S. What kind of great fun do you have with your grandkids?
100 Free Things To Do With Your Grandchild (free download)
Grandest Time/Fisher Price-Lots to do with Grandchildren