I designed this for my 20 month old daughter who has severe hypotonia. The problem was that as she got older and bigger it was harder and harder for her to fight gravity and hold her head up. I built this to take some of the weight off of her head and still allow her freedom of movement. This allows her to still use her core muscles and build them up slowly while still interacting with her environment.
NOTE: I have a public github repo that has the original Alibre Design files and STEP extracts. It can be found here: github.com/wantmys2000/Head_Balanceinator
If you print this out please note that I designed it for my 20 month old daughter. I can not guarantee that it will support your child's head without breaking. I offer this design to be used as a starting point for your research and testing with your child, but I can not warranty or vouch for its safety in your situation. Never allow a child to be alone with this device.