My 2p worth
The cheapest option to try, if you don't need an option where the child can be towed on the bike, is just to strap the front wheel of the child's bike to one side of the pannier rack (with some padding if necessary but I found the front tyre was padding enough) & tow the empty bike that way. I've found the empty bike can be a bit bouncy but not excessively so.
Maybe I've only seen poorly set up examples, but I've yet to see a trail gator setup where the towed bike is not leaning sideways at a considerable angle.
A better model, but less common, seems to be the Trail Angel (
Halford link - it was the first that came up in my search). That seems to be better laterally braced & the ones I've seen appeared to be running true.
The gold standard (& a price to match) appears to be the
Follow Me which leaves a pannier rack completely unobstructed, including the top. And it can be used without a rack too.
Former member of the Cult of the Polystyrene Head Carbuncle.