We use factor 35 here.  I think a lot of the time, you have to let the kids go in the sun a few extra minutes each day and let their skin become accustomed to it, otherwise if you block and then they forget one day, they get exposed and burn quite badly.  I just get them to put it on if they are going to be out in the sun for longer than the burn time.  Always on the face,ears  because that get’s more sun anyway.  Otherwise I keep them out of it and they just get a few minutes exposure at a time as I described above.  My kids all wear sun hats everywhere they go also.  The sun here is quite fierce..we have the highest rate of skin cancer due to the hole is in the ozone layer.  It may not feel particularly hot, but you can burn in minutes.