Make use of the recent H1N1 to reinforce that it's not they r dirty, but germs are everywhere, esp if they do go out in the day.
or you can just say it's hospital tell you to do it .
but to be honest, i dont tink i myself or my parents did wash our hands every single time we handle my boy when he was a newborn. but i insist that everyone who come home from outside must take a shower n change clothes before can carry or play with my boy.
sometimes too clean will backfire (kids immune system very weak cos not exposed to regular germs outside the house).
but it's good to wash, esp now with the H1N1 flu in SG already. Basic hygiene is most impt. just tell your ILs that it's for baby's safety, not saying that they r dirty or what.