Tag-a-long is a trade name (wholly owned in the UK by Paligap Ltd who import Kona amongst other things). What you are after is a 'trailer bike'. The most important component is the hitch which always the weak link on less expensive models. The hitch features a universal joint combined with a quick release mechanism and it needs to be robust enough to cope with the braking and turning forces between tractor and trailer, any play in this area magnified for the rear passenger making for a very uncomfortable and wobbly ride.
Gears are of little use, especially for very young kids, trailer bikes are geared to cruise at around 10/12mph and since kids don't have the same stamina as adults it pays to train them to save their energy for starting off and climbing hills.
Consider equipping the trailer with toe clips and straps, a water bottle, a bell and a cheap cycle computer to keep her entertained.
I highly recommend that you install a full length mudguard with flap on the tractor bike, one ride through soggy dog poo and you'll be glad of it.