Yellow stains on dd's sheets

[Bakersman]

My dd is 2.5 and sucks her thumb when she sleeps. this makes her dribble loads and her under sheet (won't use a pillow) gets a big dirty yellowy stain on it. I have to change it at least twice a week, normally more as you can see the stain coming after just one night. If it has been a few days I need to bleach the sheet as soaking doesn't seem to help. Has this happened to anyone else? What did you do? What got the stains out?
Any other ideas please?

Soda crystals in with your washing powder might do it, in a hotter wash than normal.

Or you could put a waterproof top sheet over that part of the bed - the kind you get for night-time training. I've got one that's about 2ft by 3ft, it's soft and comfortable to sleep on (I got it from the great little trading company I think).

Ooh could try them, thanks. I know Vanish Oxi stuff and Shout don't work... hope soda crystals will be better