Tricky one this!
We find WT+ hard to make work. Taxes are the same as CW, more or less, the difference in miles isn't huge either.
One option would be to book CW return for the Adults (125k Avios + voucher + 1540 tax) and then cash fare £1749.02 the child?
The middle ground would be WT+ both ways, but finding 3 seats could be difficult. (100k Avios + 2-4-1 + £1410 tax) As CW seats and Avios become available you could upgrade to CW, if you wished. Keep in mind that both travellers on the 2-4-1 have to be in the same cabin. For this reason, I think I would book your daughter with the 2-4-1 and an adult on their own if you are doing an all-avios booking.