Not all children with upper limb differences will need a different car seat, but for those who have very short arm/s, who can escape easily from the standard 5 point harnesses without too much issue, this 5 point safety harness might be the answer you are looking for.
View resourceIf you are thinking of replacing a perfectly good seat because your child is Houdini, this safety harness simply fills in the gap under the harness straps that the child can easily slip their ‘little’ arm/s through.
Definitely worth monitoring if you have a particularly inventive or flexible child, but in most cases a simple solution to give you peace of mind.