Thank you for sharing. Well, I am 31, wife is 26, kids are 6, 5, 4, 3. That is to start with.
I was living aboard at 31, IIRC. That will not be an issue. At 26, your wife will be physically and mentally capable of making
it work (with making
it work being emphasized).
I will also say that four kids is a LOT of kids on 40 feet. But it has been done and can be done. We had a lady and her husband living on a mooring ball (tough) with 4-5 kids. She even walked to the hospital while in labor with the last (or so the story goes)!!!! Tough gal!!
Some people just do not like boats. If that is your wife, you will have a tough time changing her mind. Just so that you know, Kris would never have bought into it if she had not seen me so passionate about it. Now she loves it too. It takes time and development. I still think taking some mom and dad only nights out on a small sailboat is the trick. You can build up from there. If you plan to just jump into it then you and she both may be in for a shick.
But again, I am giving you a lot of negatives. There are also a lot of positives. I cannot imagine anything matching living aboard a boat. It certainly is not a cheap way of life, but it is a rewarding one. Go show your wife the all the pics of sunsets (follow the link in my signature). That will be her backyard, every day. THere is no house that can equal it.
Regarding boat type/size, I think you may in fact be looking at a 40 foot boat. I would try and find one with three cabins and the kids will have to bunk up together. That really is not a problem for the kids. Mine do it just fine. But every kid (in my opinion) needs their own little personal space.
It does not take a lot of space, but simply a place that is all their own that no other brother/sister can get to (those are our rules). We also have a little "treasure chest" that they can put whatever they want in there (well, within reason) and the other sibling cannot touch.
I think a multihull would be great, but they are expensive and finding slippage is a chore. You may also have an issue with overloading the multihull that monohullers do not have so much of. I would not rule out the multihull, but I would certainly be checking out where I was going to slip her first as the cost of a marina/slip could be cost prohibitive. It was for us. There are several multihullers on this forum (with SD and I2F being the key ones) to discuss the merits of those boats - but make sure you also know the negatives as there are many. Everything has a tradeoff.
Stick around and enjoy the site. Sailortjk gave some good thoughts here too: you will get advice here that will be positive and negative. There are no hidden agendas so take all of it appropriately. Lot sof knowledge on this forum that can give you a pretty good taste of what to expect. But in the end, all advice is from a single person's point of view and should be taken as such. Your views may differ and you may be able to make things work that others wouldn't dream of trying. We did.