can greater anglia fine you for getting child tickets?

I’ve seen a lot of tiktok posts of people getting fined for buying the wrong tickets such as saying they have a railcard when they don’t or buying a child ticket and they don’t get caught in the moment but then years later they’re hit with a huge fine for all the offences they’ve conducted. I haven’t seen any for greater anglia and only Westmidlands but I’m guessing all the companies are the same and to just avoid it in general?

I’ve seen a lot of tiktok posts of people getting fined for buying the wrong tickets such as saying they have a railcard when they don’t or buying a child ticket and they don’t get caught in the moment but then years later they’re hit with a huge fine for all the offences they’ve conducted. I haven’t seen any for greater anglia and only Westmidlands but I’m guessing all the companies are the same and to just avoid it in general?

The railway companies can't "fine" you -- only a court can do that.
Bear in mind that it's a criminal offence to travel without a valid ticket, unless ticket purchasing facilities were not available at the station at which you boarded the train.
If you get caught with a child ticket when you're an adult, or a railcard-discounted ticket without a valid railcard (or, indeed, a railcard-discounted ticket but travelling at a time when the discount isn't valid*), then a ticket inspector is likely to make the assumption that this isn't the first time you've travelled without a valid ticket. At that point, they'll make a report to their office that will trigger an investigation.
If the ticket has been bought online, it will be possible for them to use the information on it to look at your previous ticket purchases.
If they've caught you on a train with the wrong ticket, and you have a history of purchasing the same wrong ticket multiple times in the past, they'll be interested in pursuing you for the fares avoided in the past.
Their leverage over you is essentially that they can prosecute you for the time that they actually caught you; but if you write to them expressing suitable contrition, it's likely that they'll settle out-of-court (i.e. they won't prosecute and so you won't get a criminal record) for the cost of the past unpaid fares plus their administrative costs.
(Note: much of the information above is taken from what I've read online in the Disputes & Prosecutions forum of RailUKforums. If you're in a situation where you're being prosecuted, or threatened with prosecution, for a railway ticketing offence, the regulars there give excellent advice.)
[*] Note that the 16-25 railcard has a minimum £12.00 fare at certain times: "This applies for journeys starting between 4.30am and 9:59am Monday to Friday, excluding Advance Fares. This minimum fare does not apply on Public Holidays or throughout July and August." -- source: https://www.16-25railcard.co.uk/about-the-railcard/using-your-railcard/