Re: minimum age is usually 6 months, you should be ok.
www.ncl.com/faq%2523age-requirements
Cruisecritic.com has boatloads of advice 😆 - just be aware that some of the posters there reflect the demographic of stereotypical cruisers 😲. Members can sign up for a roll call which is a nice way to get social even before the cruise begins.
I love💖 family cruising! I'm lucky enough to have sailed 🚢🌍with both Norwegian and Royal Caribbean in the past year. (Not on Epic yet though.) NCL is notably less formal but if you want to dress the family up to the nines for "Norwegians' Night Out" you will be well received by both crew and cruisers!
Your 5yo will probably have a blast. The childcare staff are excellent. They usually do fab crafts which I love to display around our cabin.
Food in buffet and restaurants all pretty good and waiters helpful with/delighted by little ones. Make sure you are clear on which are included and where you must reserve /pay extra. O'Sheehans pub is free & open 24 hrs - now 14yo DD and I are addicted to poutine plus DD loves to tell people she spent her cruise in the pub 
. They do room service too. We love the shows but they can be loud so sit where you can escape.
Be careful about selecting your cabin. A fridge would be vital for you but not all rooms have them. Read reviews and look at the deck plans to check what's directly above and below. NCL cabins are the tiniest I've seen (we are tight budget, cruising in a closet types) so be prepared and really organised. I have a couple of over the door hanging pocket organisers. One goes on the wardrobe and one on the bathroom door. Suitcases fit under the bed. All the walls are magnetic so I used to pack stuff for DD to play with when she woke up early. Now I found magnetic bulldog clips which are so useful to hang up a map, the daily newsletter and aforementioned crafts....
Oh there's usually only one power point and you probably won't get a phone signal in an inside cabin or at all at sea. A battery tealight candle (No Flames!) makes a nice soft glow instead of bright fluorescents. A pack of cheap glowsticks works too and you will be ready for the White Hot Party! Lightweight binoculars can be fun for the nosey.
You already know travelling with children is no honeymoon but you'll have a great time and before you know it you'll be cruise addicts!
(Wow this got looong!)