Families who hire on a short-term (i.e. summer nannies), part-time or shared basis have a unique tax advantage. Since employer taxes are tied to wages, these families typically have a much smaller tax obligation but still get to take full advantage of the tax breaks. Here's an example:

The Smiths hire a summer nanny and pay her $500 per week for 13 weeks ($6,500 total).  The Smiths have access to a flexible spending account at work, which allows them to pay for up to $5,000 of childcare-related expenses using pre-tax dollars. This saves them $2,300.  Meanwhile, the Smiths owe about $600 in employer taxes. The net savings for the Smiths is $1,700! 

As you can see, legal pay pays off – especially in short-term, part-time or share-care employment situations.