Loses something in translation maybe...
Literally: A mother can take care of children, many individuals. When she is old and senile the many people (children) can take care of Mother, just one individual?
EDIT: Sorry, I can't put it better than that....the meaning is probably, "As a mother can take care of many children, it should not be too much to ask for her many children to take care of her (as she is just one person and they are many) when she is old and frail"
(EDIT2: Sorry - perhaps you didn't want to know that)