Low fat milk is full fat milk that has been skimmed, so it's not that there is anything 'bad' in low fat milk as such it's just that when it does get skimmed a lot of the good stuff is taken with it along with the fat. So just occasionally if you've run out of the regular it wouldn't hurt to replace with low fat but it wouldn't have the adequate nutrients for a child to drink all the time. Especially if milk is their main source of calcium.