As young children complain and fidget at the best of times, it is advisable to make frequent hair washing as traumatic free as possible. The best way with longer hair (this applies to adults also) is to gently comb hair through before it’s washed to release any tangles first.
After a thorough wash of the hair, using a light massage technique, rinse really well. Many say “once you feel the hair is rinsed, rinse again and then it will be”. Conditioner should only be applied to the mid lengths and ends of the hair and gently combed through to distribute. This is by far better than spray in conditioners.