I am planning a trip with 18, 12, 10, and (almost 8) year old children in September 2017 and want to know if it would be possible to rent bikes small enough for the younger two to ride? They are both in the 4-4.5 foot range in height so can't quite make it on an adult bike. 12 year old should be tall enough for an adult bike.
Copenhagen runs a city bike sharing program, and the bikes are just the right size for all but tiny children or very large adults...
Now, one big caveat with my experience. Keep in mind, I was there several years ago, and visiting during the off-season... but the bikes were very poorly maintained and it was sometimes difficult to find a rideable specimen. Now, perhaps they've improved the situation or they keep the fleet better maintained during the summer. I hope so, because Copenhagen is one of the world's most bikable cities.
Tom they recently upgraded the bike share program with fancy new electric bikes, with gps installed and other gizmos https://bycyklen.dk/en/ (the project went horribly over budget, all at the expense of us tax payers). However I would guess that these bikes are a bit unwieldy for kids, but there are also numerous bike rentals that should have kids bikes as well.
Also be sure you know the rules of the road before you unleash yourself and your kids in traffic. Just like many others in Copenhagen I commute by bike every day and see a lot of tourists, who treat the bikes as a novel toy and are oblivious to the rules and the traffic around them.
I'm glad they've up-graded the bikes! Such a great concept for a city like Copenhagen.