I couldn't pin them down on an exact upgrade price sorry man. From what I can tell match by match for band 3 ST holders a ticket is 35. Therefore logic would suggest the 8 quid to buy the child's ticket would be taken off leaving 27 quid to pay... General sale band 3 is 40 quid so at worst upgrade would be 32 quid if you take that 8 quid off... But as I say the above is only my guess on it.
I asked this question the other day when upgrading for Palace. The response was to buy the children's 3 ticket package and then upgrade in a game by game basis.
I have two kids season tickets as well as my own. Both been used by adults already this season and no bother getting through. Think the stewards are more preoccupied with the ticketing system at the moment