Thank you both for your replies!
We have put the ladder on 2 bricks, with another brick in front, to make it less steep and create 2 steps. This seems to have done the job as a couple of days after posting my question, we saw them walk up for the first time! Hooray!
Beforehand, they would slide down the ladder, from rung to rung, so we were going to add more rungs, as you both suggested.
It is also a new coop so we have cleaned it out properly with mite spray as well.
Thanks for your help!