When mine was 3 I came across a short little Disney Princess rod at a garage sale for $1. She's 5 now and I took her out with it last weekend. She still loves it, still catches bluegill on it.
I think anything short and light enough for them to manage using it and learn the motion of casting, reeling, and setting the hook will work perfectly fine.
(The harder part is being patient when they dont want to touch the worms or the fish or let the bobber sit still. Or when you're untangling line from tree/pond bank/grass/moss/other poles/her own pole.)