Rail (625Wh) Toddlers and Trek Rails

[Larno]

Hi Guys,
I recently got a Trek Rail 7 and love it. I have a 3.5 year old and would love a way to take her with me on easy flat forest trails etc. There are shotgun style seats that I know would work but I think shes probably getting a bit big for those at around 110cm tall.

I thought about a tag-along style bike something like this:

Trailer Bike Tag Along

Buy online or in-store from NZ's favourite Child Carriers shop Torpedo7. Fast delivery NZ wide!

www.torpedo7.co.nz

But I am concerned that something like this wouldn't connect very well to a rail / dropper post possibly? Has anyone used anything liek this or have any other idea's on how to bring out a youngster who obviously can't ride a bike without training wheels yet. Or if you think the shotgun style seats would still work, they are quite pricy and I can't really try one first but I imagine shes on the limit already :/

Thanks!