There are a few reviewers on YouTube. Worth a watch.
Things to consider:
Where would you like to cruise? Europe, Caribbean, Antarctic? Although cruises try to stay out of bad weather it can happen.
How many sea days are there? Personally I would not want too many, but that's my preference.
Which company to use, a lot of the lines belong to bigger companies. With a 6 year old I would think one with a kids club would be useful.
What do you want on the ship? Some have a flo-rider, some have roller coasters, some have roller skating. All have at least one pool, but they tend not to be that big.
What sort of evening entertainment do you want? It might be just a singer, it might bee a full Broadway show. How is your little one in the evening, could they cope with some entertainment?
You need proper cruise insurance, being taken off by helicopter isn't cheap.
What sort of dinner experience do you want. Some people just use the buffet, most ships have speciality dining that costs more.
Also the ships have different rules about how they seat you, some will seat you on the same table every day, others will put you with other people or give you a set time to eat.
With a 6 year old I would check if room service is included, that way you can have a drink in your room / on the balcony when the little one is asleep.
Cruises can and do change itinerary due to things like rough seas, or ports closing to cruise ship so don't set your heart on seeing a particular place.
How are your language skills? Most cruises operate in English, but not all. There is a German line where very little is written in English. Costa give all announcements in 5 languages, would that drive you insane?