The advertisement was for The Temporary Moms Agency; a secretive and prestigious organisation that had sprung up to provide a revolutionary service to families. For some kids, their parents couldn't always be there for various reasons. And so "step-moms" would be created to serve out a limited time contract, their form altered into some maternal figure who would temporarily be their legal guardian. Therefore when parents were away on military service, or for trust fund kids whose parents didn't want to take the time to raise personally, a new "step-mom" would be assigned to look after the kids.
It promised lifetime benefits, a hefty salary and all expensed paid while momming. The idea was you'd be transformed, raise someone's kids until all of them were at a pre-agreed age, and then be changed back to enjoy some years with your wealth and perks, before repeating the role again with a new family.
Of course, in practice, only people desperate for work would do something like this, where they have to go live some other life for months on end, even years...