Hi,
The hotel next to Burj Al Arab (the six star hotel) has a water park that is open for anyone (not just hotel guests). It has many pools, slides etc. and is great for kids.
You might also consider taking a jeep ride into the desert- you can also choose to ride a camel if you wish, might be pretty exciting for a five year old.
It might also be a good idea to choose a hotel with it's own private beach and pool rather than one in the city. There are lots of hotels with such facilities.
A trip to the gold souk is always fascinating, even for a child and even if you don't plan to buy anything!
However Dubai can be very hot ( I believe also even in April) and the sun is very intense so you'll need to keep your child well protected!
Also while there are no laws in Dubai concerning how women need to dress it is always best to dress modestly. In the hotel and on the hotels own beach you can dress as you would in HK- bikini and all! However once out in public it is best to stick to trousers and long skirts. T-shirts are fine, sleeveless tops are not! Like I said there are no laws about dress- but if you wear anything that shows your shoulders and legs people will stare and stare and stare! Plus it just doesn't feel right.