don't worry about transporting it - if you already have or can fit a 12volt power outlet carry the thing deflated in a bag and when you want it out blow it up with one of these
which will pump it up in about 2 mins.
If the kids are the only ones who are ever going to get on it - then a 48" will do - if there is going to be any adult participation then you will need 56"
I actually have a 56" used once donut which I bought ready for this season that I could sell on for around £50 (including the inflator) as I have 2 kids and I've bought a 2 man triangle thing second hand from the local paper.
You can see it being used about 2/3rds of the way through this video
I was being very gentle as it was November and my son only had a 2/3mm wetsuit on so i didn't want to make him fall off