Hiya,
sorry for the verrrrry slow replies. We've since sold the seat, I think it was a maxi plus.

We do similar to Ingmar; Steca samba rack on the front (cheaper, and more functional as you can drop in a basket and it wont wriggle about, plus it has a light bracket); two panniers at the back attached with cord. Depends on the brand of child seat you have as to whether the panniers will fit between the leg supports.  I like the Ikea system!

We had child seat in the back position with no caboose until recently when youngest hit the limit. It was never a problem and kids don't need to get used to holding the caboose; they'll do it instinctively. It's probably the opposite; try to get them used to just sitting in the caboose without some form of death grip. Especially when the weather is cooler and the bars are chilly. They can drink, eat (I know, not ideal; but it shows how relaxed they are if they want to snack).
If your daughter is 3.5 and at the max size for the seat then she'll be OK in the caboose with the bars at the narrowest fitting. You can always drop a lap-belt in if you're worried about her sliding around.

Are you in Europe?
Maybe they'll ship?
https://www.marktplaats.nl/v/fietsen-en-brommers/fietsaccessoires-overige-fietsaccessoires/m1734734339-caboose-voor-radwagon-2018-2019?previousPage=lr

(on a RW4)
Steca samba. Rock solid, bolts round the headtube, rated to 15kg and its easy to drop crates in because of the lip. It sits forward of the headtube so there is space to route all cables between it and the basket/tray.

https://hollandbikeshop.com/en-gb/branded-bicycle-parts/steco/steco-front-carrier/steco-front-carrier-headset-mount/steco-front-rack-samba-44x32cm-headset-attachment-black/?country_id=261

Hi all,
New wagon owner here. Still waiting on a couple of accessories I wanted to at least be able to take one of the kids out.
Loads of talk about which child seats fit, mostly the yepps. We have a bobike seat and it fits well. I rotated the bottom clamps 90 degrees to fit the base plate to the rack. The seat clamps in tightly. I can lift and shift the back end of the bike holding onto the child seat. The seat would also fit at the front of the rack, and I could have used the downtube brace instead of clamping to the rack, but back of the rack is good for now.
First ride with the youngest got a thumbs up, especially as she could lay her legs out on the rack. In full repose!