I don't know the exact research studies, but I'm sure they're out there. I think that even just common sense.... I have families who have to make a decision on whether they can afford to take the afternoon off work to take their kid to therapy, or whether or not they can afford their child's medication, choosing between rent and medication if they don't have service, but I think just even accessing....
One anecdote is that I worked with a young adult years ago, and I remember that one day.... His parents were in survival mode. They literally were going day to day trying to get food on the table and trying to provide the basic necessities, and this kid was very engaged in therapy. I remember that one day he walked 20 minutes in the pouring rain, on his own, to get to his therapy appointment.
Kids want it. They don't always have the opportunity to get it, and the barriers are just huge when it comes to the poverty aspect. Nutritional food and the ability to get out and do active living is sometimes very difficult for families.