Vacation Bible School
These mini summer camps on biblical themes can be effective catechesis and community building, but even more importantly they can be one of the most effective ways to invite and evangelize young families into your congregation.
VBSs are generally offered to provide a service for kids and families already within the parish, but what if you thought of them primarily as a way of bringing inactive Catholics or others to the Church. The offer of summer childcare can be practically irresistible to parents. Simply getting seeker parents physically present at the parish and meeting parishioners is a huge opportunity.
If you offer a decent VBS, your parish families will enthusiastically spread the word outside the parish. The more compelling the program (free, fun theme, strong precedent from the previous year, free dinner beforehand, etc.), the more motivation and chance of success you will give them. It's also a big opportunity for grandparents in your parish to get their grandkids to come. Be sure to encourage your congregation to get as many people in as possible.
Obviously, if many families come who are not active in your parish come to your VBS, hospitality is crucial. If you are inhospitable, they will not return. But also be sure not to come across as needy or clingy. Just be friendly! Hopefully your parishioners do this naturally, but it could be good to remind your parishioners beforehand.
Be a little cautious if a curriculum was developed by someone in another denomination and review the materials carefully first to ensure that the content is compatible with how you understand Scripture and the faith.
As Marriage Support
Having a big block of time every night for a week without kids is a rare chance for couples to have time to focus on each other. Many couples defer their relationship while getting through the early parenting years, so the childcare itself can be a big ministry to parents. Encourage them to use the time well!
Getting Them Back
Don't be too in-your-face, but be sure to invite parents/families to come to Mass the following Sunday, and let them know about any upcoming events that could be appealing to them. Consider intentionally scheduling an event afterwards for this purpose (e.g. parenting or relationship workshops, with childcare). Invite them in written form on handouts sent home with the kids, but also make sure your parishioners know to invite them personally. A short note about baptism or first communion availability also makes sense, as that is often in the back of the minds of inactive Catholics, who have an ambivalence about their kids' Catholic identity and initiation.
Get Them in the Sanctuary
One church ends each night of their VBS in the sanctuary singing fun songs with the kids. Parents arrive and can sit down to watch the enthusiasm before taking them home. It's a nice way to end, but it could have special significance to some parents. It may be the first time a parent has been in a sanctuary in years, and it can be an important step in them coming back to Mass.
There are obviously pros and cons around different scheduling options. VBSs are usually held for a week at a time, but can be held during the day or in the evenings. Evening meetings have the advantage of not competing with other day camps or child care. Evening times also increase the likelihood that both of a child's parents will be able to do pick up or drop off (and thus have an experience with the parish).
You Can't Beat Free
If you look at VBS as a tool for evangelization, it makes little sense to charge participants. There are ways you can keep down the financial costs, if that will make a difference in offering it gratis. Companies would look at this as a minimal cost to "make an impression" and potentially gain a customer. We're interested in gaining disciples.
Keeping Costs Down
The Vacation Bible School industry churns out polished themed packages with associated supplies, which cost a pretty penny. T-shirts can add to the cost (although they can also serve the evangelization effort). But you don't have to spend a lot to have a fun VBS! We saw one small church put on a VBS on a shoestring budget and the kids had a ball. Especially if it's the difference between offering it for free or not, consider the less expensive approach.