I have four kids and you'd have no problem watching them for a few days ... they're obedient, yet questioning ... civil, yet fun-loving ... rambunctious yet non-destructive.
Each of our kids was different and their personalities were by no means easy to deal with, but molding them into who they should have been was our job as their parents. A few days with your kids might be hard but once they understood our boundaries I think you'd see a big difference in them. A few months with your kids and I'll bet that we'd love them too.
You assume, without evidence, that his kids are troublesome. You overlook that they could be perfect angels, but that he still feels they are a larger timesink than he would have liked. You validate the taboo: he dissents, so your immediate kneejerk reaction is that he must be a bad parent with unbehaving kids.
Here's a thought: one can be a model parent with model kids and still feel that life without kids would have been better. You can even enjoy your life and your kids while still feeling that way.
Each of our kids was different and their personalities were by no means easy to deal with, but molding them into who they should have been was our job as their parents. A few days with your kids might be hard but once they understood our boundaries I think you'd see a big difference in them. A few months with your kids and I'll bet that we'd love them too.