It sounds entirely like they initiate the contact and you just tolerate it. Not a basis for friendship, but they clearly see you as friends and I'm not entirely sure why, if they are making all the effort. Sorry if that sounds harsh, but can you re-read your own post and see how it's written as if you are interested in them mainly because "they would do anything for us and love our two boys." You don't mention whether you and they have things in common, or share interesting conversations, or have the same values, or -- anything.
Assuming you don't really mean to give this impression of "we are really only interested in them entirely on our own terms and because they would do stuff for our kids," then start to initiate contact with them yourselves. Be the ones to holler over the fence. Be the ones to call and ask them over for a while, or to invite them to some local fair or event. If you initiate more contact, you then have more freedom to be busy and unable to chat when they casually stop you and expect you to have time to stop and visit.
If you think this over and decide you really don't want to spend time with them for their own sakes and for their company, then be too busy to stop. "Hey, good to see you. Glad to see you got X done to the house like you planned! But we're on the way out the door to Child's event--catch you later this week." Be sure the story is a true one. I dont' like the idea of lying that your husband has an anxiety disorder etc. as someone posted; the neighbors might actually be concerned for him and you'd worry them.
You are incredibly lucky to have neighbors who are around, who seem to like you and who are actually willing to help out. Our neighborhood is falling prey to developers buying up houses to tear down for new ones, and we're now surrounded with lots being built on or waiting (for years) to be built on. I hope we get some neighbors who take an interest. I know too many folks who have had serious problems with neighbors -- all-hours parties, crime, drug suspicions, etc. -- and your problem isn't really a problem compared to that.