Both me and Ds1 are on pay as you go with o2 ,we have a sim that gives us 300 free text per month with a £10 top up. This works really well as we both use the text then run down the credit, so really only top up every two months.
At the moment o2 have a really good bolt on for families which would be good for all three of you. You pay £7.50 per month and nominate five o2 numbers and it gives you unlimited, calls, text and picture messaging between the numbers.
Ds has his phone topped up by me because I wanted him to have a phone when he started secondary school in September as he goes out of catchment. The deal is though that I will only top it up by the £10 per month and if like this month when he purchased lots of ring tones he has to wait until top up time for credit again.
A bolt on is a deal o2 offer such as the one above, or it could be one you have for calls while you are abroad etc. You are not tied into it, they just take the money from your credit and you can pick and choose if you want them or not.
Clear as mud me
Not sure about Car phone Warehouse, have you looked on Money Saving expert? they have a really good phone section in the forum.