I have some clippers from Amazon that have an adjustable guard on them. Similar to others here, I use a shorter setting for sides, longer on top.
I'm too lazy to get out scissors, so if I want to try and keep it longer than what the guard allows, I will just take the guard off, collect the hair between two of my fingers and cut it off at that level. Leaves a bit more length, is overall roughly even. Works for toddlers, works for me.