How to convince nanny that 3 yo needs harnessed seat still???

I'm a nanny myself and I have friends that are nannies as well. Since I am such a big believer in ERF and extended harnessing I bought all the car seats I have and the families I work for paid for some of them. My friend is a nanny but the family she works for just gave her car seats for the kids. The kids are just turned 5 and 3 year olds. They both ride in the Alpha Omega Elite 3 in 1's. I know, horrible seat! J is 5 and he is very mature for his age and I believe he will properly sit in a booster. My friend, the nanny has been asking the parents to buy her new seats aka boosters because she is tired of the 5 point harness. The parents said they would but haven't gotten around to it yet. I need help on how to tell her that A the newly turned 3 year old would be safer in a 5 point harness still. Her brother can buckle her in when they get in the car so it doesn't even require my friend to do anything but check to make sure she is buckled properly. I don't see why it's such a big deal that she stays harnessed. I want kids to be as safe as possible when they are with me because they are not my kids! I know J has definitely outgrown the Alpha Omega and A probably has or is close to it. What cheaper harnessed seat should I suggest she ask the parents to get for A? She is big for her age. Turned 3 two months ago, is around 40" tall, 40 pounds and wears a size 4-5 clothes. Maybe a Meastro?

I would show her Belle's gift. A Maestro would probably be good for the 3 year old and a Turbo for the 5 year old. It's very likely he has outgrown the harness and the AOE should never be used as a booster.