I've built a lot of stuff for kids & if they're going to beat it up,
they will. Personally, I usually build book shelves out of pine if I'm
going to paint them. I like 1x10's as the 9 1/4" width will take most
books & often leaves some 'junk' room in front. Take a good look at
your kid's books - their kind can vary in size a lot more than most.

As for sagging, 30" or less is OK unsupported, also happens to fit in
with other furniture widths pretty well - kind of a standard width.
Much longer & it can look a little weird - your call, of course. You
can also pick up support from the backing or put vertical blocks in.
Vertical blocks the same width as the shelf are actually very nice to
have. Breaks up the long shelf & can allow for neater storage. Too
many isn't a good idea, though. You wind up with pigeon holes.

Sagging immediately with books isn't your main concern. It's what
happens after a few years of humid summers or when the kid is sick &
the humidfier gets set next to it - at least, that's my experience. We
had a cheap bookcase of particle board that got ruined when the
humidifier got kicked in the middle of the night & turned on it. The
edging & plastic veneer weren't enough to save it or some of the books.
I have some other particle board shelves that over the past 5 or 6
years have deflected a 1/4" or so over 28". Looks like crap, but they
were free - worth a more than I paid for them.

Jim