I really like my Fly Creek UL 1 but if I was going to add a person (even small) I'd look at the Fly Creek UL3 instead of a UL2. The low ceiling in these tents means the slope of the walls is fairly steep, so although the footprint of the tent is generous the overhead space is not.
I would still pick a double wall over a single wall tent for younger kids because they will generally be less aware of damp walls than an adult.
The vestibule in the Fly Creek is small, so if you have a lot of extra gear to store you may want to pass on that design. I rarely (if ever) have gear in my vestibule, so a large vestibule is just extra weight for me. Something to consider, especially when deciding on a double vs single entry shelter. A single door/vestibule will save some weight. In most cases with a young child, you will be getting up when they do anyway, so a double door really isn't necessary IMO.