My wife and I will be in a similar situation in the future and from what I've read/thought about I'm leaning towards a Taco/Frontier and a full size Montero 98/99 gen 2.5.
Currently we only have a 1 1/2 year old girl, but plan on adding a few more down the road. Owning two XJ's will be a little bit rough once we have the second child. I'm 6'3" so anyone that sits behind me doesn't get much legroom. Fortunately, we plan on spacing the kids out 2-3 years apart so we shouldn't have two space consuming infant type seats at the same time. We currently use an infant type booster seat that can go from 5-50lbs and has a variety of positions that go from more of a recline to almost straight up like a true booster seat.
We had a 96 Tacoma access cab before having our little girl, but sold it due to lack of a backseat, hence the 2nd Cherokee. We don't do any serious off-roading so I'm only sporting a 3" lift on 30.5" tires. I used to have the desire/plans of having a 4.5" lift with 31/32s all decked out. Realizing that that would only cause me to go on harder trails, my plans soon changed seeing as how I'd have my family with me. Without having a proper roll cage, I don't want to be on any trails where I could potentially break something or get us stuck.
So... my decision on a Monty + midsize truck:
Having a larger family requires more seats. Monty has a third row, albeit not a huge amount of space but with spacing the kids out, the smaller ones can go in the back where they require less leg room.
Hitch basket, rocket box, or small pull behind trailer can accomodate the loss of space utilizing third row seating for longer trips that require more gear.
Due to not needing a big lift/tires, a fairly stock Monty can accomplish the things we need on the trail and the types of trails we'll most likely be doing.
IFS isn't the best for off-road, but our trails will be fine with it. Plus it'd be nice to have a big cushier of a ride on longer trips.
Factory locker with the Monty
MPG is roughly the same, if not better (from reports on here) than my current XJ averaging 15.1mpg w/mixed driving =/
Now for the Taco/Frontier... we run a lawn business during the summer and miss having a truck for hauling purposes. Originally I thought a F150 would be nice to have, but realized that we wouldn't utilize the capacity of a full size truck as much as I thought to warrant the extra loss of MPG.
Either truck technically fits 5, but realistically it gets cramped with 2 car seats. The middle part of the rear seat becomes almost useless, but by spacing our kids out in age, we shouldn't have two big car seats at the same time (unless we're blessed with twins).
If we ever need extra seating in the truck, I like Box Rocket's idea of adding two bucket seats into the bed with a topper on. This would allow for a family of 6 to comfortably be in the truck.
A full size truck would be great for space, but doesn't fit our needs. My in-law has an F150 and I feel like I have a ton of room when sitting in the back, but they're also 5'0" and 5'1" so their seats sit forward a lot.
If contemplating on a Gen 1 or Gen 2 Frontier or Tacoma, the Gen 2 is definitely the way to go in terms of rear seat space. I've looked at Cars.com comparing both Gens with one another to see which is the better one to get, and the Gen 2 of both makes is better on space.
As for the double cab Taco (gen 2) having more rear space than the double cab Frontier (gen 2), I'm not 100% sold on that unless you've tried to fit carseats in both.
According to Cars.com, here is a comparison of interior space of the Gen 2 Frontier and Gen 2 Taco double cabs
Front legroom 42.4”
33.6" Rear legroom
40.0" Front headroom
38.7" Rear headroom
55.6" Front hiproom
58.0" Rear hiproom
58.3" Front shoulder room
58.3" Rear shoulder room
101.1 cu.ft. passenger volume
Front legroom 41.7”
32.6" Rear legroom
40.1" Front headroom
38.5" Rear headroom
53.6" Front hiproom
55.2" Rear hiproom
57.7" Front shoulder room
59.3" Rear shoulder room
98.9 cu.ft. Passenger volume
The Frontier comes out on top in just about every category with regards to interior space, even if it is a slight advantage. When trying to work in carseats, ever little bit counts. Not to mention that used Frontier's are typically a little bit cheaper than used Tacos. Each truck has their own little quirks, but which vehicle doesn't?
We don't really like Suburbans or Expeditions so those are out of the question, hence settling on a Montero and Taco/Frontier. I've also thought about the newer Pathfinders, as they have a third row too... just haven't done enough research on them.
Finding the right vehicle for a growing family that you like and for outdoor recreation sucks =)