If you are a parent who would like your kids to learn some self defense techniques but you are on a tight budget, don't lose sleep because you still can. There are many different ways to get very cheap or free self defense techniques for your kids. You can search for group circles who practices self defense or martial arts, join the group then teach what you learned to your kids and as you get better, you can pass the techniques to your kids. If you are in a really tight position, watch action movies, martial arts movies, self defense how to's in YouTube or buy or borrow a book. Study the techniques, do lots of practice and observe safety procedures. Find which style suits you best, learn the pros and cons of the style you want to study but don't forget to also think/predict/know what your kid may be good at and learn the style which you think is best for you and your kids and practice together. You and your kids may never reach black belt levels or learn fancy techniques but it is better than doing nothing. Learning super fancy cool self defense stuffs will not guarantee anybody that they will be victorious in all real world fights. Often very basic techniques can bring assailants down if done properly (just like an effective sucker punch or kick to the groin...).
I love playing "play wrestling" or "play sentai ranger" with kids. I always take the role of the bad guy and get my ass beaten by the kids. The kids will not learn real world fighting skills but it can be a good exercise for both of you plus you being the bad guy, teaches kids about the some morals that bad guys will always be bad guys and the good guys always wins, plus it is really fun.
Another good way to train is to make or buy a punching bag. You can also build a makiwara or use a book or a pillow to train your kids. When my son was young, I use to hang old tires as a punching bag. Also used old car seats for his knife training and even let him try bashing several old car windows with a baseball bat (I sell cars and had lots of junkers). The fun thing he learned about hitting a car's window with a baseball bat is that the glass doesn't break and will only hurt his hands because of the shock which thought him how to select proper tools to get the job done. Using tools for training like punching bags will teach them how to properly hit without breaking their wrists, plus will increase their imagination on how to effectively use the techniques you have passed them.
When the time comes that you finances are stable, then you can enroll your kids to proper martial arts or self defense schools or probably your kids can also join MA circles or groups when he gets older...