We do the same thing - just holding their little bottoms in the warm water, and yes, ours would drift off too. So cute!I've simply help them firmly but gently and dipped their little bottoms in warm water. They have actually gone to sleep while doing this. Help them there long enough to soften everything so it would come off.
A little ACV (apple cider vinegar) in their water works wonders for pasty butt. My last couple batches of chicks I've put a teaspoon of ACV per quart in their water. No more pasty butt's.
(use an organ, raw, acv as found in your grocery stores health food section. The label should say "with the mother". Bragg's is a common brand.)
X2 - ACV with the mother is great for that! People say that feeding fermented feed also helps prevent it. I haven't done ff with chicks yet (feed it to my older flock), but will be starting on Friday when new babies come - interested to see if it reduces the pasty butt this time around.