I've been riding my Xtracycle for almost two years with two boys on the back (now almost 3 & 4 years old) using one childseat mounted to the snapdeck and a peapod behind my saddle. I have to say that while I have gotten pretty good at it, this setup is tenuous enough without adding a trailer. Look, it's a lot to manage balance wise; I got the rock steady stand and I am able to load them on the bike by myself, but just barely - there's no way I could add a trailer to the mix.

We picked up a used tandem trike last year, and mounted two childseats to it, one behind each pedaler. I could see you letting the 5 year old pedal with you on this setup, using extensions for the pedals, and when the wife wants to come along, put the youngest into the trailer. This is going to be a lot more stable for you - you don't want to risk your cargo...