Will check-in will sort us out or will they let the cabin crew provide the fix ...or will TCX do as I suspect and just split us up?
Assuming from this you fly very infrequently....
The cabin crew will not provide the fix, nor will check in deliberately split you up.
From the TCX website....
Passengers who have not pre-selected their seats will be allocated seats automatically. We will do our best to allocate seats together, however this is not guaranteed.
I suggest at check in you simply ask for three seats together for the adults and daughter, and two seats together for the boys...unless you are the last pax to check in, there should be no problem.