You did the polite thing that any father would do. However, I believe they have the right to do that if it is posted with the sign that is required by the state. A 144 sq. in. sign that states no firearms permitted.
McDonalds does not have that policy that I am aware of. I have carried there before without any problems. I would ask to have the name and number of the store owner, not the manager and ask for McDonald's policy in writing. I would think they are like Starbucks- they abide by the state laws.
Keep us posted on what you find out.
If you name the location we all ought to drop by and buy a coke!