If it was easy, would you value it?
Thank you for starting this thread. It feels like I've been living the other half of your situation, with just the two kids. It is not my intent to criticize, just to try to explain what's working for us, and as far as I understand it, why.
I started building with one four-year-old girl and a 10-year-old marriage.
Little kids are pretty intense. I didn't realize how much my parents were making up as they went along.
Our marriage goes in phases - everything clicks well for a while, then there is a time of friction. We're still learning about each other, and both seem to keep learning. It's never been easy or natural - we've both had to work at the marriage. When it's going very well, we relax and don't work quite as hard - thus the friction. I attended the 60th anniversary party for an aunt and uncle about a year after we were married - I got each alone and asked the burning question; they both independently said that the key to staying married was "after about a year I figured out that I couldn't win an argument with <insert the other's name>".
I often play outside with the kids in an odd manner, in the summer time.
A chain link fence gate protects the open garage from kicked balls (mostly), and allows verbal communication while I work on the project. If I hear "Cool Dad Come See" or other request, I go play for a while.
The kids are now 4 and 8, we're married 13 years, and I've been very happy to have them help with airplane building. One of the 8yo's friends can't wait until he can come over and help again. In the winter time, we will work for a while, take a short break, then go sledding on a hill in the neighbourhood park. Home for hot chocolate, and some more airplane work or other household project while the kids play. The kids work short periods, and get to play lots. They colour pictures of the airplane, each trying to outdo the other's crazy colouring. Then they tease me about them. I pay the kids in M&Ms, about 1 per 2 minutes of productive time.
My wife has not and will likely not ever be much help on the build, but she realizes that I'm happier with lots to do, and a project on the go. Before the airplane it was woodworking. She knows women whose husbands golf, fish, watch sports, or drink too much, all to extreme. When she gets to thinking how crappy it is that I'm out in the garage for a couple hours here and there, she thinks about that. She often feels overwhelmed by the magnitude of the project, and the cost, but I've also caught her talking about it/me to her friends much more positively.
When she rings the doorbell button I put inside the back door of the house, it sounds in the garage. And I get kissed, or cookies, or coffee. Or come running to spell her off with a stinky diaper, or whatever. We found that this started hardly any fights, compared to storming out and yelling "get in here" at me.
She knows that she can push that button any time, but I'm expecting trouble or a reward. One day she's going to work up the courage to ask one of the local RV-4 guys for a ride.
The Kids:
The kids weren't much help until they could stand on a stool and not throw rivets everywhere (about 3 for the first kid, 4 for the second). I am starting to use the kids more wisely - big daddy hands and little rivets are slow.
Little kid hands and little rivets are fast. I used three kids to put stiffeners and rivets onto the rudder and elevator skins - took them a few minutes. I'd spend half an hour on one side of the rudder, they (and a few M&Ms later) did the other half of the rudder and two elevators in the same time I'd used.
And got bragging rights, and something to tell about at school, and to be the only kids that knew the airplane parts when airplanes came up at school.
"That's not the tail, that's the vertical stabilizer!" is pretty stunning for a pre-school teacher, apparently.
The kids have helped dimple (DRDT-2 quiet dimpler), and are getting good at loading clecos into pliers for installation. I have two cleco pliers that they alternate loading and handing to me. Takes a lot of time out of the process, and costs me a few more M&Ms ("helper treats").
We take breaks when we finish a line, we count clecos, holes, rivets, add, subtract, spell, and learn a bit about life while we work. Stuff you can't talk about easily face to face is easily communicated while working together, it seems. I run all of the power tools, and I purchased hearing and eye protection that fits the kids (from Lee Valley Tools). The kids are learning shop safety, and how to use some simple hand tools. It may seem mundane to an adult, but it's a pretty big treat for them.
Building with Dad:
Some of my most treasured memories of my father are of us building something together. I hope my kids remember me that way too.
Mental Health:
I also get work in after 10 pm some nights, if I'm not feeling too sleepy. Sleepy and power tools is a bad idea. I don't drill any holes when tired, any more. The shipping for replacement parts was eating me up.
When I've had a rough time at work, or just taken another shot from the universe, my wife has learned to send me to the shop to build a little, or out for some skydives, or out flying - I come home once again smiling like the man she married.
I hope that this helps somebody. It's helped me. Thanks again for starting the topic.
Good luck to you in your life.