Two words... Recumbent Trike!
Tadpole trikes have two wheels in front and can easily pull trailers... or even a train of trailers.
Delta trikes have two wheel in the back and can be ganged together in train format and the -followers- can help pedal.
You will be hard pressed to pull all those little ones on any serious incline. If you're talking level MUP travel like the Springwater... it is easy... otherwise, they will have to pull some of their own weight.
Anyway, personally I bought a recumbent trike and never looked back. Best cycling money ever spent.
(Anura-Greenspeed; Trizard - Rans; Kettwiesel)
Big names, Greenspeed [Auz]; ICE [UK]; Catrike [USA]
(just google Tadpole recument trike)
Coventry Cycle works at 2025 SE Hawthorne Blvd has a great selection and test rides available.
I am not affiliated with Coventry or any of the brands mentioned. Just putting out the word here