Should I make daughter have hair cut shorter?

My 11 years old daughter has had really long hair since she was about 8. Her hair is lovely when its been brushed, but it takes a long time to brush, wash, condition etc. Her hair is naturally very thick and wavy. She looks after her hair herself but because its so knotty there are often big knots in it that I end up brushing out because it hurts so much she doesn't bother. She is head- sore so hates brushing and combing her hair. Her hair is currently a few inches above her waist. I've told her that if she had her hair shorter how much easier it would be etc, but she is insistent she wants it long even though she doesn't look after it properly. Tried to compromise aswell but everytime I say she is having some length off at the hair dressers she cries. So i'd love your opinions, do you think I should make her get it cut shorter, like a little longer than shoulder length, do you think I should have it cut short like pixie length or should I let her keep it really long, I don't want her to be upset however I just think her hair is too much to manage. Also I've consulted with my stylist and she said that she has got lovely hair but agreed that it would be much easier. Thanks in advance xxx