M.R.
We have done this and if any of the couples have kids close to your kids age, then please do invite them as well. Having an extra play date over really keeps the kids occupied, and often those friends will pitch in for the babysitter.
Remember, you're an adult couple with kids now. So be flexible, keep your expectations reasonable given your family status. No one is going to expect you to keep your kids 'out of site, out of mind.' It's rather cute when they come in, in their PJs, to kiss you good night. I would actually practice with them before hand. ;)