I have made a real mess of this thread and wish I can edit the title.

Just retested and seems my initial problem is accurate for the Barcelona Hotels I was looking at.
There are point stays available for 2 adults but none when I enter a child.
My issue here is not the lack of availability BUT why when I do my initial search with a party of 3 does it show as available.
Something seems flawed.

And then for Paris is as I described above with the party size not a factor
at least for these dates they seem to have available rooms for 2 adults in the member and standard category but not allowing points bookings.

Sorry newbie here ; but shocked how difficult Hyatt seems to be making what I thought would have been a simple search.