They have random prefixes as part of their name, for example "weird", "pasty", "doe-eyed", "pale".
They will target and try to snatch items from players below 50-60 guild levels (needs research to confirm exact levels). Above a certain guild level, they will no longer try to snatch from you.
This can cause them to be a real nuisance to newer players trying to do missions that involve them carrying items around that part of Ankh-Morpork. It is best to put all of your loose items inside of a worn container to protect them. They are generally only found around Goose Gate, the Hippo, Parsnip Place, and adjoining streets in that region of the city.
If your guild level is low enough for them to try and target you, you would need moderately high fighting.unarmed.grappling to defend against them - at least 260-270+ bonus (needs research to find a bonus level that lets you defend them, is higher than this amount!). It may take you less experience points to raise your guild level to the 50-60+ levels needed to make them start ignoring you, than to get your grapping bonus high enough to defend against their snatching.
Attacking one causes them to shout for help, alerting anyone nearby that they are being attacked, and by who.
They will have a mother or father NPC in a nearby room who will then become auto-aggressive to the player who attacked the brat, the parents being very protective of their horrible bratty children, so be careful attacking brats at a very low-level, and be ready to be auto-attacked by any nearby parents.
Note that while these NPCs are thieves, the Thieves player-guild has a policy that any Thief player may freely attack and kill Brats, because they do steal from Thief players too.