How strange they take 3 weeks to do the pics!

From a photographer's point of view (mine), I'd recommend clothes your little one is very comfortable in, and the plainer the better so it doesn't date too much. No big logos or graphics on the clothes either because when you look at the photos that's what your eye will be drawn to and you want the focus to be on your little one, not their outfit.

I still love the kinder photos my mum has of me in my denim overalls from the 80s. Kinder photos are the cutest!