No intention of victim-blaming here, and I genuinely hope the individual makes a full recovery.
That said, unless those 12-year-olds are complete physical outliers, if you're an able-bodied adult being beaten by children of that age, it may be worth taking a hard look at your physical preparedness. Joining a gym, learning some basic self-defence, or taking up a combat sport such as MMA, boxing, or jiu-jitsu could be worthwhile.