If your name is actually Brandon Perez, it would not surprise me at all if a train operator was keeping an eye on a forum like this, so they will be aware of what you have wrote here, which is essentially a confession.
However, I suspect that is actually likely to go in your favour. You've held your hands up, admitted you've done wrong, and are looking to resolve the matter (and obviously never do it again).
Wait for them to write to you, reply with an genuine apology, offer to pay any of their costs incurred as a result of your poor judgement, and I'm sure they'll happily suggest an appropriate settlement, probably as another forum member posted, in the region of a couple of hundred pounds.
Although, you really must not do it again, I doubt you will get a second chance.
The only thing that could possibly complicate matters is if this is only the first time you have been caught. If you have done it before, and not been caught, they may be able to trace more child tickets and journeys back to you, especially if purchased online or on your phone, in which case, it may escalate into fraud. I would assume that an "eticket" is an electronic ticket, so this may be a very important fact.