If the wife has offered you to communicate on a first-name basis then, as you correctly perceive, it is O.K. for you to take her up on that offer.
However, the husband may be a bit taken aback by being addressed as "Lieber Hans" by a perfect stranger. On the other hand, addressing them as "Sehr geehrte(r) Herr und Frau Schmidt" may strike them as needlessly stiff, even robotic.
A good compromise in my opinion would be to find some middle ground between politeness and informality. I would suggest "Liebe Eheleute Schmidt". Literally, "Eheleute" is "married people", i.e., husband and wife. Then continue addressing them both as "Sie", not "Du", until the husband, too, asks you to call him Hans.
Then again, as you say the husband has co-signed their last mail to you, so it could be appropriate to address them "Dear Petra, dear Hans" after all. This may be a situation where your intuition should be your guide. In any case they are hiring you as an au pair, not a German teacher, and you can be 100 percent sure that any minor language gaffes from you will be forgiven. In fact, they will probably preserve them for the family chronicles :)