Most airlines actually do not sell the front row of seats in economy for a certain amount of time before the flight because they are reserved for passengers with disabilities who require these seats. These passengers basically know to call the airline because they have a special consideration (service dog, knee which is fused, some wheelchair users, etc).
I am not certain, but if you call the airline and explain your situation, they are usually more than happy to help you. Basically they would rather have you call ahead of time and have them move seats around or other such arrangements than have to worry about it on the day of. I have purchased seats on Delta, AA, and Frontier and never had a problem. I usually purchase online (sometimes through 3rd party sites, though I do this at my own risk), then call and say that I use a wheelchair full time, I am traveling with a party, and that I must sit next to my wife and I must sit in the second row (my wife cannot handle the bulkhead row - makes her too claustrophobic - she has her own medical problems too). They are very happy to make what I need happen (sometimes its not row 2 but row 3 or something like that), but I have learned that if I call they are usually very happy to help.
This is only my experience, but generally if you call and explain they try to do everything they can to help (since they know you are bringing an autistic 6yr old on the plane anyway and they want to make life easier for everyone)