Mouthguard should be obligatory part of sport equipment, needless to say that is a absolute must in contact sports. You can get inexpensive mouthguard in most sporting goods stores, yet it offers the least amount of protection. For a proper thing you should visit your dentist and make one there. Most important thing is that mouthgard feels confortable in the mouth, cause if not, it will probably stay in your or your kid sports bag.
For full teeth protection you can get your mouthgard at your dentist in just a few days and for a price around 30€, which is a small price considering the damage you may have to fix when not having one. Also, we would recomend to start wearing mouthgard in the young age so the kids would get use to it, and it could become usual part of the training gear.