Hi! Sorry for the late response– your messages got caught in our spam filter.
So the northern lights will be tough in April. Not impossible, but very difficult. Take a look at the twilight data here:
https://www.timeanddate.com/sun/iceland/reykjavik?month=4&year=2018
Note there are three different kinds of twilight: Civil, Nautical, and Astronomical. These are just different levels of dark. Something like:
Civil: Dark. Outside of civil twilight, people can’t really see outside anymore to do stuff without a flashlight.
Nautical: Very dark. Outside of nautical twilight, you can see some of the brightest stars
Astronomical: Super dark. Outside of astronomical twilight, you can see lots of stars
(The data is a little confusing, since this shows hours of daylight; you need to consider the times outside of the ranges given, since you want darkness.) So by mid-April, it never gets “super dark.” It’s only “very dark” from maybe 11 PM – 3 AM. And it’s only even “dark” from 10 PM to 5 AM.
You can see the northern lights during civil twilight, though it might be less impressive. But you’ll be up very late or very early. In my opinion, I wouldn’t choose to go in April just to try to see the northern lights, especially with a 6 year old.
There will be fewer tourists in April, though!