Janet:
If you're looking for a Moderate resort that would have a theme aimed more for a 4-year-old, I would consider
Caribbean Beach Resort. Its pirate-themed pool with water canon stations as well as its
pirate-themed rooms fit the bill for young adventurers. The weather in March can be mild enough to enjoy the pool, so it would be a solid choice.
Although considered a Value Resort, another recommendation is the newer
Art of Animation resort that includes well-themed rooms that highlight favorites from the movies
Cars, Finding Nemo, The Little Mermaid and Lion King. You can request which room you prefer or upgrade to a suite to give all of you more room, not to mention an additional bathroom and common area. The pool at AOA features playful sculptures and water stations along with zero entry. The resort does not have a table service restaurant like the moderates but if you're looking for kid-friendly, I believe it may exceed your expectations, especially if you choose the suite.
Wishing y'all a magical vacation!
Whitney