L.M.
MY MIL bought one for the bed my kids use at Home Goods for $20. My almost 4 yo daughter has been potty trained for some time too but has accidents at night (too often!) occasionally. Sometimes none for a month then sometimes 3 in a week! Put pull ups on on top of her underwear, so she feels it if she pees but does not make a mess in the bed, just a suggestion.