Without more information, it's hard to say.
If you died, who would take care of your kids? Would any child support continue?
If you died, since they are still young teens, they would probably get Social Security benefits, which can be surprisingly generous. You can check your Social Security record to see what they would get. Since you have three kids, the family maximum might come into play.
If any child support and any Social Security plus your employer's policy wouldn't be enough, then you could add more to get them either to age 18. Some might pay to get them through college, but I'm guessing you are not planning on helping them through college if you live, so I wouldn't allocate resources to making things better if you die, but some people would feel differently.
Just sit down and figure out how many years you want to provide for them for, and then multiply by maybe $50K a year (which is what you're probably living on). Maybe a bit less if they would go live with a grandparent who already has a large enough house.
If you need more, I'd just look at one of the online brokers like selectquote.com and get maybe a 10 year term policy from a good insurer. (It sounds like this may be what you did with the $20 a month policy.)
Sometimes there are pricing breakpoints, so a $250K policy might be about the same price as a $200K policy.