Child stoker kit
"Child stoker kit" is the official name of the assembly that allows an extra crank to be installed for the child. If adding this and handle-bar extensions and so forth, brand new parts will easily exceed the cost of the tandem itself. Evaluate the usefulness of putting this on the bicycle carefully and try to anticipate the use you will get out of it.
I would tend to push you into waiting for a deal on a bike similar to the one that was stolen, that you were able to use without the kit. You will likely find this to be a more economical option and one that you know already has worked with your family in the past.
That said, I put a child stoker kit on my triplet and find that it was well worth the money that I put into it, but YMMV.
Enjoy your family rides now while you can. Kids grow up fast.