Fodorites,
Let me put your (collective) mind(s) to rest!
I have booked with Butterfly Balloons (a company recommended by Esbelli Evi where we are staying). They dont allow anyone under 6, partly for safety reasons and partly for not spoiling the serene experience for fellow travellers. So my husband and I are doing this on different mornings.

Gretchen - i wish the baby sitter was an option. But when my daughter wakes up in the morning, if she sees a stranger and not one of her parent, she will get really upset.

Meadeparsons - Without getting into a debate about 'responsible parenting' in this travel forum, part of the reason me being OK with this was due to a friend's experience in San Francisco. She took her 1 year plus daughter for a balloon ride in napa valley and the experience was amooth for everyone

Napa Valley and Calistoga is right in my back yard. Most companies require children to be 7 years old. They all require them to be at least 48 inches tall. Current California insurance regulations require them to be 48 inches tall also.
You never know if your landing is going to be smooth until you do land. Winds change in a moment.
Like you, I don't want to debate responsible parenting on the forum. We can all make up our own minds.
Have a good trip.