Hora, get a weeride.
I got given a Hammax rear mounted one, it was OK did the job, but did splash out on a weeride in the end and wished I had just binned the rear mounted one.
The weeride is far superior, nice having him right there between my arms, manouvering the bike up the side of the house with him in the seat is much nicer as the balance is just right.
Your a bit late though, probably only get a year out of one now.
Mines on a 26″ BMX, & you will struggle fitting it onto a 14″ MTB, I bought a 2nd weeride 2nd hand and had to modify the bracket to fit wifes bike.