Our children's SPF30 sun lotion offers 97% coverage from UVB rays. SPF50 would only offer a 1 percentage point increase in protection. However the SPF number only relates to UVB rays, not UVA, and sun lotion needs to be re-applied regularly to maintain protection from both.
The danger in giving a higher SPF factor gives false confidence to stay out in the sun for longer periods. The UVA rays have a deeper affect in the skin cells and you cannot see the damage this is causing (the UVB rays are the ones that cause the burning on the surface of the skin).
We recommend that you re-apply sun lotion every couple of hours to maintain protection against both UVA & UVB rays.