Yep! We just went this past August! We took the boys pirate costumes down, but it was so hot that they ended up wearing their PotC tank tops and then we just put pirate bandanas on their heads with eye patches and big gold hoop clip on earrings. There were a lot of kids dressed in the costumes, but they looked awfully warm! Going in April could be a little different as far as the weather goes, but if I were you, I would go with a cooler back up plan!
If your Son is into pirates he will have a blast! IF you want to meet Jack Sparrow I would suggest either going directly there when you enter the party or wait until right before the end of the party. The line was almost a 2 hour wait when we walked by!
The Magic, Music & Mayhem Fireworks are NOT to be missed! The best fireworks display that WDW has ever put on IMHO!