Samsung tablet wrongly added to my Sons bill

[BombayBob]

I recently upgraded my wifes phone and got a tablet on a 24 month contract in my local O2 store. My wifes phone usually come out of my bank account. My sons phone is also on my account but he pays using his own bank account. 

Now it seems when the O2 staff set up the new contracts they accidently added my wifes tablet to my sons account. Who is now stuck paying for the airtime and device plan bills.

I have tried to get the tablet added to my bill but O2 have said because the number is linked with my sons I would have to pay the full amount to get it off or just pay my sons bill for 24 months.

I don't understand why you can't just unlink the number and add it to my account.  

[MI5]

@BombayBob 

With O2 it is impossible to unlink accounts but it may be possible to change the billing details in your MyO2 under manage my details.

If not, your wife could set up a standing order to your sons account for the full amount each month.