I was changing a child I noticed blood in their butt and stool I have a 4 day old cut on the knuckle part of my thumb and a scratch with scab HIV risk

Why? Unless you suspect that the child has HIV, there is no risk in transmission. If the child indeed has it, then it will depend on how well healed the cut on your wound was and even then the risk of transmission is likely very low. Nonetheless, in case of doubt, get tested.