Cheap options, since you bought your camera off craigslist etc. and I assume you don't want to throw a lot of money after it:
1) The best brand name prime for you on a budget is probably the 135 f/2.8 softfocus. Can be had for $200-400 used if you look a bit, and is optically great. The softfocus can be turned on or off, and you may even like it for portraits of kids, but even if you never use that feature, it is a good deal.
Get this if your kids are moving a lot and/or you are photographing them in low light buildings or dusk/indoors, etc. (since it has AF to get them sharp while moving + wide aperture for low light)
2) Get an old manual 135mm prime lens and an adapter. They made probably millions of 135mm f/2.8 prime lenses back in the day, and for instance, a pentax can adapt to your camera with a $5 adapter ring. If you get a clean, lightly used copy, it will probably be optically just as good as a modern 135mm prime, but instead of $300, you pay $45 for an
ebay lens and an adapter. Do have to use manual then, though.
Get this if you are really strapped for cash and are willing to train manual focus skills in exchange for saving $$.
3) Get a used 18-135 IS STM lens and just completely replace your kit + extra range. Most expensive option probably at $400 minimum, from amongst these. But it is a great lens that can meet all your kid photographing needs. It is not as fast as the above options, though, being f/5.6 at 135mm (2 stops slower). But it has the fastest AF of them and the most convenience with zooming.
Get this if you usually photograph your kids staying relatively still and/or in daylight or really want the convenience of a zoom.
OR
If you are in fact swimming in cash and I judged incorrectly, then get a 70-200mm f/2.8 lens for the best of the convenient somewhat long range portrait and action lenses. Canon's are all great. Or save a lot going with Sigma and get a 70-200 f/2.8 with image stabilization for $1000 used. A 135 f/2 at also around $900 is an alternative luxurious option for extremely fast moving kids and/or low light.