If you're lucky, your actions could result in just a single fixed penalty notice (which requires a payment of £50 within 28 days, or £100 if you leave it until after that).
However the education authority could choose to issue a separate penalty notice for each day that your child misses school (so that's £250 to pay, or £500 if you leave it longer than 28 days).
Fixed penalty notices don't create (or add to) a criminal record but a court conviction does. The education authority is under no obligation to use one or more fixed penalty notices. They could choose immediate court action. The maximum fine is one of £2500. A community service order could also be imposed upon you. (Theoretically you could be sent to jail for up to 3 months, but no court would consider such a penalty unless a child was regularly missing school and the parent was condoning it. A parenting order is also a theoretical possibility but, again, it's unlikely).
In addition to any action by the education authority, you might find that the school would suggest that you find a different school for your child. (That was always the policy of the school which I taught in, under similar circumstances).
http://www.direct.gov...areAtSchool/DG_066966
Chris