I always double or triple layer sheets. Mattress > Mattress pad > sheet > mattress pad > sheet them maybe even a third layer for a kid who is sick or potty training, etc.. That way in the middle of the night I can just strip the top layer and don't have to remake the bed, kwim?
And my final comment, I potty trained DD at age 2, it was awsome! I'm so glad I didn't wait longer, and actually wished I had started even sooner. I read the book "Diaper Free Before 3" and higly recommend it to every parent!