Just lie him down on his back and very carefully u can wash his hairs

Maybe water is going in through the grommets. I can imagine that would be distressing for a child and the surgeon may not have ever thought about it being an issue. Maybe temporarily keep shampoo or whatever you are using away from her ears. See if you can temporarily keep her hair clean enough by wiping it over with a damp facewasher / flannel until the grommets come out. They may only take a couple of weeks to come out and the ears to heal. My understanding is that them grommets are supposed to drain the ears of stick stuff and they are getting water in them when you wash her hair. That is the only thing I can suggest.

Have them lie back like if they were in a salon

I used ear plugs and one of these special ear covering swimming head bands for my youngest and made sure the bath water was only a couple of cm deep

I used to put blue tac in my son’s ears. I’d make it thick but didn’t push it in too hard. So long as it blocked the opening only it was fine.

My kids don’t have grommets but we use a shower hat thing that lets the water run off away from them, kind of looks like the ruffle a clown would wear around their neck. I just slide on top of their head (ours has press studs to adjust) have the hair out of the op and was and rinse, no water gets anywhere but on the top of their head and it runs off like a water fall, most chemists have them