Pee drops on toddler's bed

[Koteburo]

Hi guys!

Well I had a question today from a cousin:

She lives in Italy, she has a kid about 3 years old and a 4 year old female cat, spayed. Lately the kitty cat has been going to sleep to the boy's bed but the problem is when she gets up she leaves a few drops of pee in the sheets, which means changing everything and washing every time she does it.
Recently my cousin changed the cat's bed by the heater and she seems pleased with that but eventually she'll go back to the boy's bed and do her pipi thing.
What could this possibly be?