A group of people with disabilities is not always a group in which everyone has equal power.
Some examples (by no means an exhaustive list, and not all of these examples apply all the time):
People who are better at asserting power can have more power.
People who speak more easily can have more power.
People who think more quickly can have more power.
People with the most common disability in the group can have more power.
People used to being ok with their disability can have more power.
Power dynamics in a group always need to be monitored and taken seriously.
Restricting the group membership to people with disabilities can be part of the solution, (because it can eliminate the part of the problem involving nondisabled allies and parents taking over), but it can never be the whole solution. Power dynamics exist in all groups, even with members of the same marginalized group.