Jake, a friend of ours' little boy has Fly boots (oh, sooo cute), he's somewhere between ages 2-4 (heck, I can't remember), but only rides pillion so far ;) Obviously I have no idea of what size that translates to. I think Fox may make toddler boots too.
As for you getting clucky watching Brandon ride, I know that feeling, but then once the little ones are off the bike, tired, hungry & grumpy, I don't want to know about it. I'll just keep riding with the ones I ride with & once they're too fast, I'll find some others to adopt for a couple of hours a week/fortnight/month ;)
Thanks for sharing your day, it's way cool & your nephew's cute (want to trade for mine, only thing is he's 14 & not into bikes at all - basically no use to me whatsoever hehehe - don't tell my sis that).
Also, keep an eye on the for sale forum, as sometimes the little boots do come up, otherwise you could post in there ;)