You need to do some more research NOT on DCUM to determine what benefits you want/need so you can negotiate. Typically: 5-15 federal holidays, 3-5 PTO days (sick days, dr appts, sister's graduation day, etc), 2 weeks vacation (1 chosen by family, 1 by you). Health care stipend is a higher level perk (I didn't receive it when I was new to nannying but I do now) but you should try for it anyway.
If it's 45 hours a week 5 hours will be OT ($30/hr) so make sure MB is clear on that.
$20/hr in the Bay Area is not bad - pretty average actually. If you feel good about the family and the hours and the benefits I'd say take it!