Firstly, how much does your child weigh and how tall is he?
Second, do you have a specific budget? Is it $30?
The BubbleBum is definitely legal in the United States, and is properly crash tested. It would likely be an option for your situation (some people get them just for traveling or carpooling), however there might be better choices. The BubbleBum is quite small, and it therefore might not provide comfortable support and it’s also prone to becoming ‘tippy’ for the same reason (we looked at one for carpooling situations for our daughter when she outgrew her harnessed seat, but decided against it).
Ideally, at five years old, a booster might not be suitable. Clearly carrying a heavy car seat around is far from ideal, and traveling is tricky with any sort of child seat, but have you considered the
Ride Safer Travel Vest? It’s probably a bit more than your budget, but if you’re going to make similar trips in future, it’s probably good value for money. It’s fairly compact, and will easily fit in a typical suitcase or travel bag.
The
IMMI Go seat might be worth looking at as well. Like the Ride Safer Travel Vest it’s not cheap, but will provide something much closer to a harnessed seat while still being very portable. And the same applies regarding budget – if you’re going to make multiple similar trips in future, it will become much better value over time.
The other option is the
Mifold which acts like a booster but is much smaller. It’s not expensive – about $30 to $35, but the fit of the seatbelt is not always consistent and depends a lot on the car you’ll be renting. Also, of these options, this is probably the least preferable choice for a 5 year old, due to size and fit issues.
If a low backed booster is the only feasible option, then the
Harmony Juvenile Booster might work. It’s cheap and doesn’t tip like the BubbleBum. We got one for our daughter for carpooling and it’s proved adequate, although it’s not that well cushioned.
I would perhaps look at the
five-step test to determine if a booster is the best option, and then consider that with regard to your travel plans/situation. There are other options than the above – including a harnessed seat if possible – but hopefully these suggestions are of some help.