Please don't design cycle infrastructure for children. Bikes can be toys, but they can also be so much more. Designing for children locks bikes into the toy-role. Do I have to arm myself with a car to be treated like an adult?
That does not mean design a two tier infrastructure, that is kids using shared pavements and real men riding with HGVs. Take a universal approach that enables all ages and all abilities to cycle safely.
"Really, you have to take that nice, scenic, ten minutes detour because I would not let my child ride the direct route. If you're in a hurry, take a car". This is not how you make people switch to bikes, this is how you teach them that bikes are merely toys for a nice Sunday afternoon ride.
I'm fine with designing for humans, but not all infrastructure has to be child proof, as the interstate network clearly shows. We have the technology to create motor vehicle infrastructure ready for unattended children (bumper car floors, yay!), but for various reasons we chose to take very different tradeoffs.
Two tier infrastructure can by the way also mean that children up to a certain age legally share the sidewalk, which is a much better place than sharing even the most segregated bike lanes with texting pedelec pilots.