Couple things- Don't ride a fixed gear with a kid in tow, thats putting him at risk. If you are planning on freewheeling on a single speed then I dont recommend a 15 tooth freewheel-there is only 1 brand made and it sucks hard.

Here is what I do for a situation similar to yours:

Run a flip flop hub- freewheel both sides-or at least the side you will ride with your son on
Use a 16 tooth for yourself, use an 18 tooth with him in tow

run a double crank- Pick a chainring that works for your 16 tooth for the outside chainring, and get one that is 2 teeth smaller for the inside.

My frame is a single speed cross frame that I ride road in a higher gear ratio and off road in a lower ration. At long as you have the same number of teeth on both drive setups your chain can be 1 length. If you want to run 1 chainring you will need a chain tensioner unless you have really long dropouts/track ends.