In your situation I would look into 20" wheels. I am one of those who built several homebuilt longtails and it is not something you want to do in an apartment after work when you are a parent.

Haul-a-day is exelent but since the child is 5 yrs old I would think it is time to look into a Bike Friday tandem or similar.

Also there is "trail-a"bikes that you atatch to the adult bike and the child can do some pedaling. If you use a folder or a 20" bike and atatch a one wheeled unit for the child the whole thing is easyer to manage.

Lovely Bicycle!: A Haul-a-Day in Ireland