I take it you've never been any significant amount of time in Copenhagen. There a bicycles for transporting children that are made to transport up to two children, usually up to 3-4 years old, after that they can cycle themselves so I've never heard about it being an issue because the eldest will usually be old enough to cycle.

edit: I guess you could increase that to 3 children if you attach a bike trailer, but those aren't very popular here.