Is it ok to ask parents (members, kids in CM) to volunteer in order to help out?
Definitely. But I would probably ask them to serve once a month and then figure out a schedule that works.

Is it better to have ‘fill-in’ volunteers who may not feel called to work with kids or cancel the program until people step forward?
If it’s really bad, from a quality standpoint, I would cancel it. But, if the quality isn’t really bad then I would work with the fill-in volunteers while actively recruiting new ones.

Does it do the children a disservice to have volunteers there to ‘fill-in,’ who may not feel called to pour into their lives?
It’s a disservice to both the children and the volunteers, BUT, it’s okay short term. Some of those volunteers may have never volunteered before and this will help them find out where they’re niche is. The best way to figure out your passion and gifting is to try stuff out.

Do the people that serve every week need a break (to go to adult service)? & If so, when? Should a CM worker’s forgo time off to go a service for the benefit of the program?
With one service, you could try to get people to serve every other week, but you’ll need twice as many volunteers. If you’re church is all about reaching the unchurched, then continue to cast that vision and let people know that your church values creating an engaging environment for unchurched people over churched people going to service. That’s definitely not a long-term solution but it’s how I would want to do it. I would make sure the volunteers get a CD of the message, or the whole service, each week.