My opinions, not as a qualified mechanic:
If you have an automatic transmission, and nothing wrong can be found, and you're on top of maintenance (replacing ATF, inspections), drive the van until the tranny breaks or starts slipping, then trash the van. Automatic transmissions are basically black magic. You can buy remanufactured ones from the factory, but I would never trust any shop - independent or dealer - to fix the internals of an automatic transmission unless you have absolutely no choice. Most fixes to one are temporary at best, and the thing starts dying soon after. The only exceptions are certain models that are relatively repairable, coupled with very obvious problems (clutch rings wearing off their friction-y material, that sort of thing, which is obvious to eyesight and doesn't need special diagnostics.)
Can you put a kid in the front seat of a Fit? Depends on their age (really, their size.) If they approach adult-sized, sure. If not, no. No, you can't go turning off fucking airbags, that's not okay. Your kid is not going to be better off without an airbag, you're just trading one harm for another.
Buying new vs used is an age-old argument. If you plan on buying a relatively inexpensive, reliable model used, and driving it into the ground over the course of 10+ years, it becomes a much closer decision. Especially if you live in an area where weather doesn't ruin cars. Depending on where you live and what you're looking for, you might find the depreciation on currently for sale used minivans to be not nearly enough to convince you to buy used. Or you might find used to be the obvious choice. Do your research, and create a rough spreadsheet calculating total cost of ownership (purchase price + taxes + any special up-front payments, then maintenance over the term, then gas, extended out to the life of the vehicle.)
