Size them large enough to wear socks. They add a bit of slop, but my Veloces in a comfy size with light socks only hold me back on 5.10 and up vs. my Miuras, and even then only on tiny holds. The socks help even out pressure and provide some cushion, so much that I prefer the same shoes w/ socks instead of without.
Painful shoes are for projects. Until your kids are working on that 5.11c, comfort comes at a low cost.
Edit: oh, and kids grow. When/if you buy them shoes, get them as big as possible without seriously hampering their climbing. The snuggness will come slowly, and they may appreciate it eventually.