Should I take on another commitment?

My schedule is pretty packed: I work 40 hours a week, at an office to which I commute an hour each way. When I get home, I work on assignments for grad school, and I'm taking a full course load of 10 credits. This summer, I'm trying to secure an internship that would be for about 10 hours a week, and I'll be taking at least 3 credits in addition to that. I sing in my church choir, which is a weekly commitment of 4 hours plus one additional rehearsal a month (and possibly some outreach/concerts). I'm planning our wedding for October.

So why would I want to add anything else??

The church music director asked me and my sweetie if we'd consider joining her husband to start a chapter of SpiralScouts. He has a few interested kids, but needs a co-leader or two both for paperwork and for practical administration. The Circle would meet once a week (Sunday mornings?). I'm actually considering it seriously.

Reasons for:
I want to spend more time with kids.
The reason I visited MCCSF in the first place, back in 2006, was to see if I could help out with their Sunday School program. Back then, the congregation had enough families with children that this was a regular thing after the morning service. (I ended up joining the evening service because I liked the music better, and I didn't want to wake up early for 9 AM rehearsal... especially when singing in the service meant I couldn't help in the kids' room.)
I think SpiralScouts is a good cause to support. Youth organizations like Girl Scouts and Camp Fire Boys and Girls can teach cooperation, independence, leadership, and other necessary skills for living in society -- not to mention giving parents a break for a bit AND including them in activities when desirable. They're open to children of all faiths, which is important to me personally and to our church. And there isn't one in San Francisco! Closest Circle listed on the website is in Richmond.

Reasons against:
I wouldn't get to sleep in on the one day of the week I usually can.
It takes about 90 minutes to get from home to church, depending on public transit. It costs about $15 for a round trip. I'd be doing that anyway on Sundays, but...
Meeting in the morning during or after church services means that I'm effectively stuck out in the world allllll day Sunday, because choir rehearsal is at 5 PM and we get out around 9 (remember, 90 minutes to get home after that).
I would have to pack lunch and possibly dinner.
I wouldn't be able to leave town for a full weekend. I already have to be back in time for evening rehearsal in most cases, but this would short that vacation time by another half-day.
The uniforms are really dorky.

Neutral or mitigating circumstances:
I could take my laptop and work on homework, etc. using the church wireless connection. Or the music director offered to have me in her home (help with kids? fold some laundry? take a nap? read a book?) for the time in between.
I'd need to have a background check anyway to start foster or adoption procedures in the next couple of years. I've had a few for various employment involving minors, but nothing recently.
We might be able to work out a system where I'd only have this in-person commitment once a month, which is much more manageable than every week. But we might NOT -- or it might fall through, and then what?
During the summer, I won't have the monthly responsibility of the Read Aloud program. That doesn't balance out the new internship responsibilities, but it does offset them a little.


What do you think? Am I crazy? I feel bad saying "I'm too busy" to a woman with multiple part-time jobs and four children under the age of 7. I'm sure she gets it. But I'm trying to balance a really neat opportunity against an idea of how much is too much.