"It doesn't seem to worry them so why does it worry me?" Exactly! perhaps that is the question you should be trying to find the answer to..... There child is coming to no harm, and I imagine they are loving parents, and will bring their children up well. I don't like to preach, but i thing they will have well adjusted children without any hang ups. All to often I speak to people who can't stop cleaning up, and just wish they didnt feel such a compulsion, they just want to have fun...
On the health side, it is actually not a good thing to have a perfectly clean house, for a start all the cleaning products used today are probably more harmful than a bit of dirt to young children. When a child is kept in a super clean home they are not exposed to all of the little bugs and bacteria that you find in a normal environment. There bodies never learn to defend themselves .
On a more straightforward note, it's not your house, no-one is coming to any harm, so don't judge.
When my children were babies, my mother in law used to come to babysit one night a week. She was a spotless tidy person and made me feel quite inadequate, I used to hide the ironing in the cupboard or she would do it for me. One day she opened the cupboard and it all fell out on her head..
I laugh about it now .. but was horrified at the time.. there is nothing worse than feeling like someone is judging your skills as a housewife and mother..