Hey OP, I had a bit of a lightbulb moment last night about this

With my dd's they would freak when they saw the bottles of shampoo and conditioner, let alone when I attempted to wash their hair.
But last night I was thinking (whilst washing dd's hair) I wonder if the kids would still react the same if you got some of those clear or coloured sauce bottles, decorated them, then filled them without the kids knowing with shampoo and conditioner. When hair washing comes round you could grab it and call it something random and add a quick story to it, perhaps they will play the game and let you wash their hair?

Just a thought anyhow? Im tempted to give it a go with mine to see if it makes a difference.