Some parts of the UK have fluoride in the tap/drinking water, in these areas it's advised children are given fluoride free toothpaste so that they don't get too much fluoride (which I believe can cause health issues). Other areas the water doesn't have fluoride so it's advised that toothpaste with fluoride is used from the get go to help protect teeth.
I know some people are against the use of fluoride full stop but that's more a personal/health choice thing. I'm just talking about the guidelines in the UK re fluoride - HVs that tell you what toothpaste you must use, then give you one they've advised against. Our dentist also advises adult toothpaste with fluoride for LOs regardless of age too. It suits me, no separate toothpaste cluttering up the bathroom and no having to change from kids flavoured stuff to adult minty stuff.