Poplar with paint and clear poly would be a better
choice when it comes to moisture and kids.
Might even think about laminate for the surfaces.
You could ust matching or contrasting colors. like
black with white laminate.
You can also get MDF with laminate already on it.
You can buy the laminate in narrow strips to do the
edges for a nice finish.
Remember, the MDf is very heavy. Ply is also an
option. Still needs protection for moisture.
MDF is also rough on cutting tools.
How long are the shelves to be? Anything over
about three feet with books needs some center
Now you should be totally confused.....
Build a boat while you are thinking about it,,,^^^^