Getting cats adventure ready

Advice/ opinions pls :)

I have two 4 month old kittens and I want to get them use to being on a lead and harness so we can go on safe walks when they are older. So trying to build their confidence to new places and get them use to the harness.

I am visiting a family member who lives an hour from me and was thinking it would be a good opportunity to get them use to some travel in a car and exploring news places ( a house not outside). But worried in case they are too young.

One of them is super confident and is fine with the harness and I think would be fine travelling. The other one is quite nervy and only likes being touched when is sleepy so not got to putting the harness on yet and I'm more worried about her not enjoying the experience.

Any advice on how to get her more confident would be great too. At the moment I just try to hold her for a few seconds a day extending each time ( but rn only about 10 seconds ) and stroke her whenever she comes for cuddles at night.

Thanks in advance!