I've seen "urinals" that were just tiled walls with tiled catch basins. You could get some used / scrap / cheap tiles and make up a little urinal that drained into a small bucket and then empty the bucket occasionally. How much can one little kid pee anyway.
To beat the odor, I was once told a little sugar in the bucket will deal with the scent. Certainly I've used a bucket while camping and the smell, when using the sugar trick, was inoffensive.
Peeing outside is a male right. Discretion, of course, being the better part of ...