The crucial element for fitting is that you need at around 15mm of spacers under your stem for the “stinger” (attachment) to fit, without this you will struggle. Also high rise riser bars will provide the most comfort and ease of getting little ones in and out. A stem of aroun 90 – 100mm is also necessary. If you can tick all those boxes then bike fitting is relatively straight forward. There may be exceptions to the above but that has been my experience. Currently the work horse bike is a Dialled Love/Hate.
In terms of child size then my youngest has been in our one since she was able to wear a helmet comfortably. she is still riding around comfortably now at 2 3/4 (around 90cm tall) with her feet not close to touching the bottom of the chair.