Rather than take DD's convertable booster & DS's hipod, I've decided to use the hipod for dd & take a half seat booster for DS.

With the half booster, am I able to check it in for a flight alone? Or should I put it in a bag?

I'm not sure if the carrier I'm using checks weight for kids items or not. I think you're allowed one item & I'll choose the stroller if that's the case. Car seats are lucky to weigh 5kg.