I'm quite close to my MIL and not long ago she helped out quite a bit with mine and DHs child when they were ill but we desperately needed to go to work still.
As a thank you I got a little present made from a local seller and it was engraved to Grandma lots of love X (our child's name) and have given it to her alongside her main Christmas gift from us all. Item was bought solely from my own money (me and DH have separate disposable to buy what we like with after bills ect).
My husband has older DC too (large age gap, our DC is in nursery, DSC in secondary).
AIBU to have gotten a separate gift for MIL from just our child?