Hey mamas with older kids, when did they stop using a booster car seat? My LO is 7. We are now…

Hey mamas with older kids, when did they stop using a booster car seat? My LO is 7. We are now expecting baby #3 and I don't know how I'm going to fit 3 car seats in my vehicle. I was wondering if I could eliminate the booster seat but of course I would t want to sacrifice my child's safety. Any suggestions are appreciated!!!