Those Brussels in April will notice that the air is quite pleasant at 13C in the daytime. Nights are actually pretty chilly at 5C. The 18 days of rain will probably give the impression that it's actually colder than it actually is. Luckily, the daylight will last for about 6 hours, giving more time to explore the city.
Easter Monday marks a giant egg hunt around Brussels' parks. If you travel with your kids, make sure to take them to this event they won't forgive you if you don't!