Just thought I'd answer my own question in case it helps anyone scrolling through.
I've my child's renewal info come through (2 days after the previous claim ended) and the rate has increased from middle to high, and has been backdated to January (when I sent the claim) so if you have renewal forms through and think there may be a change, get them sent back ASAP!
Due to current workload, they have also set up payments from my child's 5th birthday month which falls within this claim period, for low rate mobility as well, so no need for me to do anything after he's 5.