A quick capture as we've had many people ask how we carrying a kid's bike on our bike or back when either kid tires out or when dropping off a woom kid's bike rental (yes we rent kids' bikes!) to a customer.
I am using the woom "balance bike" carrier strap. I have successfully carried up to a 24" lighter weight Prevelo kid's bike with this. There is also the option of a tube or tube with carabiner but it isn't nearly as quick or comfortable.