1) Go Zoo, it's a very good destination for the family. If you go on the last day before you return HK, you can bring the luggages with you and safekeep at Zoo which is only around HK$1x per luggage. Reasonable price.
I also booked Zoo's breakfast which sounds nice & special.
2) USS - many people says you may skip it given you only has 4D3N. Yep, been there 1.x years ago too, and the main attraction to me was Sesame Street's shows. As the tickets are not too cheap, you may skip it.
3) Vivo City, if you don't go for Food Republic, there're are some more choices as the mall is big. This time I plan to go "Marche", which is a Swiss restaurant with kids play area and nice kids menu.