Is a child 3 years of age free for both attractions?
Or send them an e-mail, they are very quick to answer
Call the number on the burj khalifa official site and they will give you a sure answer
Be sure to book tickets in advance. I used Viator. They will send you a second Burj confirmation . That’s what gets you into the tower. I was fifteen mins late to my time slot but they were very nice about it. Aquarium has no set timing on it. There is a boat ride to enjoy also. Look for a combo ticket.
Per the At The Top Burj Khalifa booking site you’re three year old is free “infant”. Enjoy!