OP, I'm going to PM you tommorrow, if that's ok. If you'd like a hand, I can give you some specific recommendations but it'll require make/model of your current seats and the kids sizes

(Somehow I missed their ages in the first post and thought it was two families travelling together!)
In general -- depending on the seats and double stroller, you can often place the carseat in the stroller and bungee it in (putting the bungee through the belt path and attaching it to the stroller frame.) You could also use a luggage cart -- check the stroller and bungee the carseat to the luggage cart, sometimes that's easier, or use a travelling toddler to attach the seat to a carry-on. I would probably take the 2 & 3yo's seats onto the airplane, and carry on a backless booster for the 7yo, which can stow under his/her seat. That leaves you with just the baby and the 1yo as problems... plus of course the rental vehicle, lol. If the baby is still in a bucket, you may be able to stow it overhead and install it baseless in Orlando. Maybe invest in an
RSTV... it's essentially a wearable 5-pt harness, the 3yo could use it at your destination, and put the baby in the seat the 3yo uses on the plane? That will also help you fit 3-across the third row of your rental van, since the RSTV is only as wide as the kiddo wearing it.
So, plane/car:
7yo - airplane belt (backless stowed under seat)/backless booster
3yo - convertible seat A/RSTV
2yo - convertible seat B/convertible seat B
1yo - lap/convertible seat A
baby - lap (bucket stowed overhead)/bucket installed baseless.
The RSTV packs flat in luggage and the bucket would probably have to count as a carry-on but with five tickets, you probably have a carry-on to spare. And while the RSTV is expensive, a) it may be less than the price difference to go up to a large van, and b) with the size of your family and ages of your kids, I think you'd get a lot of use out of it for future vacations.