Found the Tesco sim only contract deal works well, buying phones separately.
Dd1 (15) has 500 mins, unlimited texts and 1gb data for £10 a month.
Added DD2 (10) with 250 mins, unlimited texts for £7.50 a month
Added DH, same as DD2.
My phone locked into O2, but will consider when replacing, my usage would be same as DD1.
So currently £25 for the three of them, would be £35 for the four of us.
As more than one family member on same account each get an 'extra' - either 150 mins, 250mb data or 500 Tesco mins IIRC.
So DD1 and DH have 150 extra mins, DD2 has the data (not a smartphone, but can access Internet)
Also have DD2s 'capped' so she cannot go over the allowances, which also prevents premium rate numbers, calling abroad, picture messages and anything else not included.
All on a rolling month, so can get out quickly if better deal around or if they were to abuse it.