I guess this topic has been discussed here but I find it hard to navigate through all the useful information in the site:
My twin girls turned 4 yesterday and I am not sure if I can use seat pads and backrests, or do I still need Thule Yepp Maxi for them? They are a bit small to their age and weight about 16 Kg each.
Thule's website says 40lb limit. 16kg ~ 35 lbs.
https://www.thule.com/en-us/child-bike-seats/rear-mounted-child-bike-seats/thule-yepp-maxi-_-12020214
My experience is that my 21 month old would not hold on very well in the Caboose but she loves being in the seat. But I'm very confident with my 3.5 year old back there. He'll never ever let go of the caboose supports. Its up to you I guess. If you think they are comfortable not being the seat, then go for it. I think the caboose is awesome and a great way to keep them safe in case of a tipover.