Language teaching on a freelance basis can bring in some cash - my wife recently started teaching her native language via a local college. The rate isn't bad - about £15/hour - and they pay one hour's prep for one hour's classroom time. At the moment it's all been online due to COVID, but I think there are plans to reintroduce face-to-face sessions in the new academic year (however, since they can now recruit teachers more widely, and attract students from across the region, they may keep an online option as well). I don't think they offer ESL teaching in that strand, but I know they have English classes for international students arriving to do their courses - a kind of booster to get from the minimum IELTS standard required for a visa to the advanced level of English needed to actually complete a degree. If you have a uni or college near you, it might be worth making contact - most of these courses operate over the summer so new students are ready for the start of the academic year.
The difficulty, though, is that this is not very flexible. Sure, you can prep at 2am if you want, but you have to be available to teach without interruptions at a set time each week. Our problem was that my wife's lesson times and my work deadlines often clashed, meaning Disney had to babysit our 4yo. Not ideal, but we just about made it work.
Also, the work is seasonal - term time only, or set up in anticipation of the academic year. So you can have a profitable few weeks then a long lull before the next round. We're already looking at expanding the business, working on the principle that there won't be thousands of people clamouring for my wife's language, but if we can find half a dozen individual students, we have enough to create a full timetable.
I'm also self employed as a journalist / editor / translator. This was going well until COVID - I was clearing £30k fairly comfortably and effectively working part time (odd hours, specific deadlines, but not all that much time in action and an almost free summer if I wanted). However, I was well established in my niche before going fully freelance (and my niche is an odd one, relying on a fairly obscure language and an interest in a minority sport). There are other ways in to this, or bits of copywriting, but it can be a battle to gain experience without a huge amount of work for little or no money - possible when you're 17, difficult when you have family of your own.