big tangles in kids hair are the worst! It make you just want to cut them right out of their hair! but here are a few tips:
#1 realize its going to take a long time. You can put your kid in front of the TV while you work so that she doesn't get squirmy.
#2 put A LOT of conditioner straight into the tangle and rub it in gently (careful not to tangle it more) leave it there for a couple of minutes so it sinks in good.
#3 using a wide tooth comb and starting from the bottom with one small piece at a time, gently work the knots out. If you hold the tangle with one hand at the top it will lessen the amount of tugging that she feels.
Good luck!

P.S my mom made me wear my hair short until I could take care of it myself