I sometimes take care of a child that has hiv. can i get it by accidentally touching his poop and urine?

Very unlikely: Hiv is primarily transmitted via blood and bodily fluids in very close contact. Poop and pee have less virus.However it is best to use disposable gloves and wash hands thoroughly after changing a child's diaper in your care whether they have HIV or not. If you are in a daycare setting there are many other bacteria and viruses that can be transmitted more easily without proper hygiene.