So I do understand your concern, there are very unstable and desperate people that you will be working with, and in the case of CPS you may be intervening and taking their children. The foster agency I interned at had several clients make threats against staff. So in my eyes your concern is justfied, however your hands are also tied in a sense.

For one thing a knife is useless, and if you ever stab someone to death that's a career ender. Honestly I think you would have a better chance of explaining shooting someone than you would stabbing someone, it's just a very hideous and primal way to kill or injure someone, and the odds of you doing this well enough to be effective and actually save your own life are slim.

Most agencies have policies about you carrying weapons, but not all do. Depending on what state you live in, you may have the option of obtaining a concealed firearm permit. However, even with such a permit, you may not be able to carry when you are working because of the agency policy or state law, it would be more for your personal protection off the clock I guess. That's the extend of your options for lethal weapons, a CCW permit, which you probably can't use when working.

The other option that makes I think the most sense is pepper spray, because this is legal in most states if not all states. I would keep it as a last resort, but if you pepper spray someone who is attempting to hurt you that's going to go down way better with your employer, the licensing board, and the police than will stabbing/slashing that person or shooting that person.

Tasers for civilian use sort of suck, they aren't really all that great because the moment they aren't touching the person it's useless. That's why pepper spray is good because it hits their face and sticks to it, making it hard to breathe or open their eyes, and snot pours out of their nose. This gives you time to get away.

Your primary means of dealing with this kind of a situation is planning. If you are going to a dangerous call, see if you can have police assist you, or at least not go alone. I think typically if you are taking someones kids that's done with the police. If someone comes after you, it's not your job to get beat up, run away and call the police. You get a weird vibe or think you are in danger, break contact and get the hell out of there, talk with your colleagues or call the police.