Portrait lens for Grandchildren and picky parents

Some background info: I've got a three year old son and I am shooting lifestyle family sessions professionally.

I guess there is no perfect solution or answer to what you're asking. Each decission has its pros and cons. I will include in my comments only lenses that I have or have used intensively (Disclaimer: next paragraphs reflect my own opinion only. Not "true" facts)

23 f/1.4 - Excellent for environmental portraits. I find 23mm a FL beautiful for toddlers snapshots. but its AF will make you miss some/many shots. Sometimes I shoot couple sessions (not family sessions) professionally just using this lens.

35 f/1.4 - Amazing image quality and slow AF. Difficult to use with fast moving subjects but nice for posed portraits.
35 f/2 - Much faster AF than the 1.4, so it is easier to use with moving kids. But if you are really seeking for bokeh you will miss the f/1.4 look.

56 f/1.2 - Again, amazing image quality and slow AF. Excellent for posed portraits and really tricky to use with toddlers. You will really have to master AF-C settings and techniques to use it with fast moving kids.

16-55 f/2.8 - Fast AF. Very Versatile. This is my go-to lens when I have to shoot "uncontrolled" sessions. See an example of a full session taken just with an X-T2 and  this lens, combining posed moments and candid ones, all with natural light only, no reflectors, no flash (turn sound on)

https://oscarlagarrotxa.smartslides.com/celines-family-in-mallorca

Nevertheless only you can decide if this looks "professional" enough to you

My final recomendation would be to stick on 16-55 and try to get the most of it.

I hope it helps.

Óscar
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