People drive on average 18 km/hour on a bicycle here (flat terrain). That is nowhere _near_ 50 km/hour. Racers drive such speeds, and they are aerodynamic, on a race bike, well maintained bike, etc.
Our oldest is now 4, like I said she can barely bicycle as it is. Kids of age 8, 10, 12 they go to elementary school, and many go by bicycle. Sure, some walk too. After age 12 they go to high school, and these are more far away usually so its either public transport or bicycle. Bicycle is cheaper for parents, and the kids get some exercise, plus the bike lanes are reasonably secure (its illegal now to use a smartphone while using bicycle). Now, yes my 4 year old can barely cycle, and she should be on the sideways indeed. However at some point she's gonna be old enough to start cycling to school and such. And you want her to get combined with cars who drive 60 past her school where they are only allowed 30? Its an insane thought, seriously. Open up Google Maps or Apple Maps or whatever and have a look at the Amsterdam center. Look at some of the roads where cars are allowed. Now, yes, the roads where people live in a house/apartment and such, these are generally small and yes the cars gotta drive together with the bicycles. But when you go from one neighborhood to another, the cars are separated. They drive on their own lane, usually easily 50 km/hour, multiple lanes, traffic lights or roundabouts at different larger roads. You really do not want to cross such roads casually, you want to use the zebra crossing. And as a cyclist what you want to do is listen to the traffic lights, and stay on the bicyclist road. This is in a large city.
Of course, in a small village or even small town, you can have it more casual. For example you could say the main road which connects to a city is mainly for cars, with separated bike lane, but for the rest cars and cyclists must share the road. And this works, perfectly well. But I grew up in a town of about 80k people, and we had various separated roads.
My point being we already make all kind of concessions in The Netherlands:
1) Bicyclists may not use smartphones anymore while using their bicycle because these caused accidents.
2) Sometimes streets and bicycle lane is separated, sometimes the bicycle lane is shared but marked as such (means cars may use the area, but bicyclists have priority), and sometimes they are shared which in practice means cars much adapt to the bicyclists (and they don't like such but if they not allowed 50 km/hour that's OK). It depends on the area, and its decided by local city/province government.
3) Electric bikes are an ongoing issue/debate. Usually the elder drive them, and they cause accidents. I mentioned it because they're a serious problem, but according to local law they're not allowed to drive more quick than 25 km/hour or they fall under different jurisdiction than manual/classic bicyclists.
(In your world, the manual/classic bicyclists would be able to easily go twice as quick as the legal limit of electric cyclists. While they can go quicker than 25 km/hour, and racers do indeed regularly, the casual cyclists do not and will not. They will not adapt to the 50 km/hour of the cars. Also, people in The Netherlands often have old bicycles ('barrels') cause they still work, and these won't get stolen. They won't have a new chain, they're made for flat/city terrain, and all too often you have wind not in the back. So its not only 4 year olds who would be in disadvantage; also pre-teens in general, as well as many elder.)