Well, making friends is always good, let alone friends in Christ, right?
Who knows later your children may find his/her partner in life in this bible school? having a partner in Jesus is always good, right?
Well, other reason, as a parent, we need to bring our children to God. There will be time where we cannot control our children and they may reject to come to God if we do not teach them properly.
I really enjoyed VBS as a kid. You learn about Jesus in a fun way. It's also good to have structured activities that are enjoyable in the summer. Good for the kids, good for the parents.
The proof is in the faces of the children who return to VBS year after year. The proof is the new families who come to know Christ because their little child told them about Jesus.
I met a lot of my lifelong friends at VBS and it really is important to children that would otherwise be spending their summers alone or in a less than perfect home. A lot of the kids at my church come from very rough homes and they really look forward to the break in theier crazy lives.