7 Seater Cars – UK's Top Selling

My 3 yr old daughter recently started with a new child minder. She takes her to school for me. She has a number of children of different ages she looks after. I've recently found out that my daughter does not use a car seat in her car. She has one of these 7 seater cars with smaller seats that fold down in the boot. My childminder says that as these seats are smaller and only to be used by children, and as the seatbelts have adjustable heights that no extra carseats are needed. Does anyone else have a car like this? Is this legal? In my car she has her high back booster seat, surely a three year old should have a car seat or are there different rules for these smaller seats? Please advise a very worried Mum!