All of my Indigos and Cribos have different (individual) personalities. With some of the sub-species, it seems that maybe individual personalities really means varying levels of nastiness. For example, I have some Yellowtail Cribos. All of them are nasty to one degree or another and don't really seem to want to tame down much, if any. I have a pair of Mexican Redtail Cribos that are puppy-dog tame. I also have some Blacktails and a Unicolor, all of which are pretty tame. I have an adult Texas Indigo that I believe, in all sincerity, if he were human he would be the leader of a street gang. I have a juvenile female Texas Indigo that has never offered to bite or really shown any sort of bad attitude.
The bottom line, though, is that I'd be leery of trusting any large snake to be handled independently (without close adult supervision) by a 7 year old. Biting is just a matter of opening the mouth, positioning the head correctly, and closing the mouth. Anything with teeth can bite and will do so under the right circumstances. An Cribo or Indigo gets very large (8 ft ) and is capable of inflicting a serious, although non-venomous, bite.