Ugh..
Perfectly legal.
You've invited someone round and paid them to fit something.
Their T&C's for fitting are that the room is clear of heavy shiz.
They arrive, room is not clear of heavy shiz - they say they can't do it or they're about to. Having been round this particular merry go round more than once, they take a quick snap to show to their boss the issue.
They refuse to do the job and to refund the fitting fee as per T&C's, you didn't fulfill your side of whatever.
You raise the issue with their boss because you're pissed and standing on no legs, and boss sends you a photo proving they're right...
But - PLOT TWIST - photo has your kid in it.
You think 'AH HA, GOT YOU, THATS ILLEGAL'...
And cue Mumsnet, drama etc.
It isn't illegal, if their GDPR shiz already covering your address, financial details, etc is fine then storing/handlign a digital image is also not an issue.
If they wang it up on FB and tell everyone you're a bellend and so is your kid - that might be an issue.
Back down OP, you have not caught them out, you have not won on a technicality.