My amazing neighbour just gave me this wagon can you tell me where you bought your harness for seat belt.For all little ones my favourite is a wagon. I have a large one from this company from when we lived in London - www.toywagons.com
What I would do is put them three to a side with legs crossed in the middle or alternate legs out - mine is 24 x48 for the bed so plenty of leg room. I used the walking harnesses to make seatbelts putting the harness on each one and then the rein part wound through the slats to prevent standing. For the 3 month old you could use a carrier or use their carseat and put it across the front of the wagon and the others behind. The oldest should be walking just to get the exercise too.
I have the foundations quad bought from a caregiver closing down. It is ok but then I am used to the wagon and my daughter's wheelchair and used an older double stroller most of the time my own kids were little so don't find it that bad to push. Don't bother going out in the snow cause it is just too hard on the old wrists to pull or push anything I don't have to as in no school runs anymore. What I don't like is the safety brake bar as you have to hold the handle and the brake bar - thin metal strip at the same time as the brake comes on as soon as you let go. Great for safety but annoying for pushing because instead of holding on to the handle and a leisuring stroll you have to grip this thing tight the whole way - solution was to tie it to the handle so I don't have to hold it. Means no safety brake but around the block on a flat street isn't an issue for me.
Cheers,
Sue