I wanted to throw in my thoughts:
I don't live in a tiny house, but we are hoping to build one soon. So I have been figuring out a lot how to do this with a growing family. It won't be a house on wheels, too pricey in australia, but small and affordable. I have given a lot of thought to this. Mostly because we don't have heaps of money, but also because we just don't use the space in a typical house. We have grown to a small family with 2 kids and we still only sleep in one room. I think the thing I have come to is to forget the dogma and the "tiny house movement" but take the inspiration to only have a house as big as what you need. There is a lot of wasted space in a typical house. I think for inspiration design the house like a small apartment. Have heaps of storage planned into it to. We have been downsizing and organising. We currently live in a 3 bedroom house and we literally only use the bathroom/laundry/ kitchen/lounge/ and one bedroom. We are always squeezed into one room together. Kids don't want to be on their own, they always want to hang out with you. I have figured out that our dream house would have one bedroom/sleeping loft, small bathroom (a small bath is helpful with kids), a family WIR/linen, an open living space, and a small galley kitchen that at least accommodates a large fridge and decent sized pantry. It can be open to the living area, but needs to be a bit more than what you see in a tiny house. I would also like a patio or veranda and would keep the washing machine out there. I guess if the kids need their own room down the track we could add on another room. I'd probably want a spot for shed storage and to keep some keepsakes. I keep a few boxes of clothes the kids will use down the track. Its not necessary to hold on to it all, but I like to keep it and save and reuse stuff, especially as I sewed a lot of it myself. I keep shoes! They cost a bomb and they grow fast so its great to have bigger shoes on hand for the next kid. At this point we don't know how many kids we want. But i aim for something that could eventually accommodate 6 kids if need be. I like what Ross Chapin has designed for his pocket neigbhourhoods. They are a good size for a family. Rather than separate rooms for everything they utilise nooks for extra space. So a one bedroom house can have a nook for a study that could be enough space for a bunk bed instead, plus a little loft provides storage or extra sleep space. Its a good approach that you might like. If I thought the kids needed their own room, I probably would only consider a little spot for a bed. They won't play on their own in their room. I also think its important to balance tight space with open space. If its all too tight it will feel crazy uncomfortable and crowded. But some spaces can be tight and well organised like the bathroom, storage, kitchen, sleeping. I guess the living space is the main thing I'd like to have open and spacious and not over crowded with storage. I'd also have some massive doors to open to the outdoors and give more space. A good indoor/outdoor connection with kids is really helpful!
I thought if the kids need more room as teenagers we can add some space. They can build their own tiny! And take it with them if/when they leave home. They can park it elsewhere on our property and start their own tiny house compound. Maybe you could have a little compound of tiny's or two connected by a veranda?
The thing about kids and stuff is not necessarily true. I don't think kids need to own more than they can look after as in pack away easily. So the baby has 2 baskets of clothes/including cloth nappies. My son has a little cube storage and one special box of treasures. Their toys fit into the buffet in our lounge. I have embraced the Konmari method of decluttering/organising. I guess if I had to quantify it we each have about a suitcase of clothes. We have 2 pairs of shoes each. I have 2 sets of sheets only per bed. A small number of muslins and washers and towels. Now, this isn't about living some sort of idealogical life of austerity, I'm not into that, but honestly its just so much easier and less work to have less stuff to keep track of. I decided we can't own more stuff than I can remember where it is, because somehow "Mama" is the master "keeper of all the things". Kids don't need stuff, and kids don't need their own room. My kids have always slept with us. My oldest is 5 now and has his own bed pushed against ours. I think I'd need a sleeping space ( without wardrobes, just space to sleep) big enough to fit 2 double beds. I'd like to get 2 ensembles ( no frame) and push them together.
When you have kids the first thing is to not allow people to give you too much stuff! Train your family and friends to give your kids experiences not stuff, a trip to the zoo etc...
The other awesome thing is the toy library. I don't know if your community has that, but mine does. My kids could really just get a new toy out each week and take it back and get something new. They don't even really need to have toys then. But they have a set of duplo and lego that gets used heaps. The toy library is great for novelty toys that you get bored with quickly or large toys that take up lots of room. We were given heaps of toys at my sons first birthday, we just didn't have the room so we gave a lot to the library. Avoid toys that a child can't use in open ended play. If its too narrow in its range of play or it just entertains,is a novelty, it needs batteries, or its flimsy plastic its not worth having.
Living small with a family needs thoughtfulness about what you have and what you do.
Check out these. A "tiny" house for a family may not be as small as the tiny's they show on the webs that couples and singles use, but they will still be real small. I have found though, that actually its not always cheaper to be crazy tiny.