My 5yr old has Down syndrome and has lots of sensory issues and one of them is washing hair. She literally goes frantic.
What i do is apply the shampoo/conditioner in the living room with a face cloth, before I take her in bath. Then we play with dolls and cups in bath. Pouring water using a cup over dolls head, my head, my daughters head.
Also those soft bath/shower caps for babies are helpful
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I talk/sing while I put the combined shampoo conditioner in, I use a shower head attached to the bath (its a rubber hose with shower head that fits on the mixer tap) and wash it out as quick as possible.

QUICKLY – and chose a shampoo that is chemical free – there are some great options at the chemist. maybe bubs has an allergy?

In short…quickly. I found the more I tried to be careful the more stressed they got. In the end a very big and quick jug of water over the head worked! Distracting and getting a giggle after was easy & it was all over with and quickly wasn’t an issue. Good luck! It’s not easy when these daily must dos are a challenge & every kid has theirs, my daughter is 9 & still freaks out when I cut her fingernails & no she never got cut! The joys huh?

oh i agree on the doll tip! that is a good one

Good tips on washing hair; little hair washing hats seem to help too.

Try to distract them with toys and lather up. Give them a little shampoo and ask them to help. If you can’t rinse it off, then wet a washer and wipe shampoo off. Give them a washer for their eyes as a barrier from the water

My daughter did too
dont fret too much. Maybe a small squirty bath toy on her head. Small cup of water. Concentrate on gentle water on head then the actual hair wash. Take her doll in bath and do hers first. Baby steps. Rather than full on water on head and big hair wash.

Depending on your child’s age. Use baby shampoo. Mix with a little water then massage onto dry scalp. Take a shower to rinse off, turning it into a game. Or in the bath give her a jug to pour water over her own head to rinse it off, without her having to lie backwards. Some children do not like the strange feeling of lying backwards in the bath.