Why paint the bottom, which will be covered with sand, but not the back or side?
I can't believe you let her take the time to brush that twice instead of going out to get a roller, btw. That was mean!
Will you be figuring out a way to cover up the sides since there's a visible overflow and the plumbing/holes? That presents a minor design challenge, but it could also just be done with a nice wide piece of fluted moulding to frame the entire unit, which would class it up quite a bit.
I also wonder how you plan to put a lid on that giant space. Kind of amazing it doesn't have cross pieces...