This is already a pretty well-studied part of anatomy. NSFW? Spooky Scary Fetal Skeletal Development Images I don't know if it's necessarily NSFW- this image is seared into my brain from HS anatomy, but remembering it made up my mind about chips and salsa, so fair warning it's basically fetuses with transparent/nonexistent skin.
Anyway, you can see the developing bones in red and you will notice they aren't even close to connected in (A), in (B) they are much closer but there's still gaps. First this is kinda necessary for birth to be possible as it is. If the bones were fully set there'd either be fractures via birth or the rigidity would rip the mother stem to stern. More importantly this development process doesn't really fully end until you get towards adolescence, this is where the "babies bounce" factoid gets its origin.
If you think of bones as steel, natal/infant development is when its at it's most liquid. Lots of give but also very little strength. As they grow those connections grow closer, bones become more rigid. Kind of like working temperatures where the steel is red and somewhat solid, but you can work it into place. Things like foot binding take advantage of this development to horrifying results. However if a child simply practices gymnastics/flexibility consistently they'll never need to train for bring back what they lost. Wrapped around a pole might be a little much but this is part of why it's a good thing for kids to be active. If they're hunched in front of a computer all day that posture can become permanent.
