The hours for April are published as 9.00-7.00 from the 4th to the 29th and 9.00-8.00 for the 30th.
IMHO SW is a great park for that age group. There is a great kiddies play area ( Shamu's happy harbour) , feeding the dolphins and seals will be popular. At that age the kids don't usually have long enough arms to correctly feed the stingrays, but they usually could spend hours playing "chicken" when trying to feed the rays

. The shows will be popular, it's worth taking in the "Pets Ahoy" show for that age group. It's a little "cutesy" IMHO for adults, but kids of that age seem to love it. With the Whale and Dolphin show, it could be worth your while getting to the stadium quite early ( 30-40 minutes before the show) , your child might be the right age to "star" in the show. They need to get a volunteer child from the audience and there will be a CM going round the front of the stadium looking for a likely candidate (AKA victim

) . If you see a CM with a clip board wandering around about 30 minutes before the show try to get your child to ask them a couple of questions about the animals. The CMs are happy if they can find a child that's confident enough to talk to an adult and interested in the animals. Couple of good questions are 1) What do they eat and 2) what do they feel like.
An easy to miss atraction, but VERY popular with this age group is "The Tide pool", it's only a little atraction outside the Coral Reef, but it allows little kids to dip their fingers into a tide pool that has fish and creatures in it. The CMs will often help the kids to hold a sea cuccumber or similar.
It's likely they will run a second day free at that time of year, with a little one you may find you need more than one day to cover all of SW. Kids can be enthralled for ages with some animals and IMHO it's a shame to tear them away when they are having a good time to go look at something else they may not like. IMHO you need to go at the childs pace and see it through his eyes.
P.S. SW is quite a spread out park with plenty of walking, if you have a stroller I would suggest taking it. Otherwise mom or dad could be doing A LOT of carrying of DS