I use factor 20 minimum, always look at the back of the pack for the 'star' rating on the UVA and UVB. I prefer to get a brand I recognise, purely because I'll know how much effort/trauma it'll take to get two children to stand still long enough for me to get it on One summer we survived purely because of the clear spray on stuff from Ambre Solaire. My daughter was old enough to know she needed sunblock, but not old enough to sit while I lobbed it on properly. Thirty seconds of fairy twirling while I sprayed and a tactical pair of Adam Ant-esque new romantic-face-paint-block stripes on her face and we were usually good to go
When my daughter is in school on a belting hot day I use the 'all day' stuff from Boots, but be aware that 'all day' is only six hours