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Our boy is 4 so he's on a small Isla Cnoc (14" from memory).
We lay down a thin sheet of neoprene over the rack (or anything to stop metal-on-metal) and put bike on it's side and then bungee the hell out of it to pull it down against the rack. His bike sticks out the back a bit but works well. We leave the canopy on the bike virtually all the time so this limits the amount of stuff we can have hanging out the front of the bike...
We fitted a rear rack on our Douze that enables us to bungee our boys bike on the back because once you chuck a kid in the front with a bag, there's not a lot of room left! Use rear rack for pannier too as it means I can use the same bag for my work stuff whether I'm on the cargo or fixed.