Sobo--
Eagle Creek is a great hike but to get back to tunnel falls its about 7 miles or so--maybe not the best hike for a 3-year-old.
Horsetail is a short hike, Oneonta will get the little guy gurgling with glee. Both are easily do-able on a casual day trip through the gorge. Taking the scenic highway (adjacent to I-84) puts you in close proximity to all the falls in the gorge and you can just pull over whenever you want.