While donkeys are not particularly aggressive, they will respond if they feel threatened. Jennies (female donkeys) are especially protective of their young. Donkeys may warn you with hissing, baring of teeth and stomping when they want you to back off. Donkeys have a very powerful kick that could definitely cause serious injury, so don't provoke them.
Remember, animals respond to perceived threats, which may not be actual threats. Keep your pets and children within fenced areas of your yard.