I think it would be a good idea to look into a professional to evaluate your daughter.
My youngest daughter is diagnosed with a mood disorder, which at age 7, sounded a lot like your daughter. NO amount of discipline, charts, threats worked and our family life was suffering terribly - so was my daughter.
I am not saying your daughter has a mood disorder, but I think it would be in everyone's best interest to find either a Child Psychologist or Child Psychiatrist to do a full, intensive evaluation to get to the root of her behaviors.
I will paste some links for you to explore, and if you have any further questions or concerns, please write back.
I believe you are right to be concerned and not just let this behavior go on and on......
This is what the American Academy of Child & Adolescents Psychiatry says about lying:
If a child or adolescent develops a pattern of lying which is serious and repetitive, then professional help may be indicated. Evaluation by a child and adolescent psychiatrist would help the child and parents understand the lying behavior and would also provide recommendations for the future.
Oh boy, this is a tricky / complex situation on several levels, but you know that already.
I won't go into all my daughters history (she was put on risperdal at age 7 due to anger outbursts)..........But, I will urge you to find a smart, qualified, board certified Child Psychiatrist to evaluate your young daughter before medicating.
If a child has been potty trained, then starts having regular accidents at school, I believe that is a sign that something is going on.........
It could be a million reasons, but your daughter may not know how to express what it is bothering her.
Has ANYTHING changed in her world at all? New daycare, new teacher, new baby, new home, a death in the family, or ????
I would take a hard look at your daughters world from HER eyes and see if you can figure out what is troubling her.
I would also ask her pedi for a referral to a Child Psychologist - someone for your daughter to talk to and explore her inner feelings, because having accidents at school is a sure way to get out of school, be teased by other children and that could cause a new set of problems for her.
I am not a professional, just a mom who has learned a lot over the past 25-years, and my best guess here is this isn't really about the food its about anxiety.
With divorce and step-parents involved, treatment needs to be consistent and collaborative. You mention the Mom doesn't think anything is wrong - it might take educating her about anxiety / ocd stuff in children to have her realize she is not doing her son any favors by not acknowledging he has an issue.
You can do some research online, talk to a knowledgeable Child Psychologist and decide how would be the best way to proceed.
I think parenting classes would be a terrific place to start. If you are not sure how to find them in your area, I would look up local Child Psychologist's and call their office for references. You could also ask local pediatrician's.
Obviously this 7 year old has gone through a lot, and he may need some counseling services himself.