As for Jurong bird park, it’s advisable to spend at least half day there. If you must go to Jurong bird park, can I suggest you visit Jurong bird park in the morning instead as Jurong bird park opens at 8:30 a.m.. This way you would be able to maximize your time in both Jurong bird park and USS. Suggested itinerary would be as follows:
8:30 a.m. – Jurong bird park
1:00 p.m. – Leave Jurong bird park and head down to USS.
2:00 p.m. – Arrive at USS
7:00 p.m. – Finish your day in USS
If you would like to focus on the attractions for kids in USS, I would recommend the following itinerary (the sequence follow the routes in the map that I embedded in my article):
1. New York – Watch the show – Lights, Camera, Action!®Hosted by Steven Spielberg
2. Ancient Egypt – Treasure Hunters kiddy rides
3. The Lost World – This is a theme area designed for kids, you can spend more time here. Show that must not be missed -> Waterworld – catch the 5pm show
4. Far Far Away – A fun theme area for kids as well. Shows not to be missed -> Shrek 4-D Adventure and Donkey Live
5. Madagascar – A theme area for kids as well.
I believe all these activities in USS should be able to cover in 5 hours. To answer your query of how to get to Sentosa from Jurong bird park, they are actually located quite a distance away. I would strongly recommend you to take a cab to avoid wasting any time looking for MRT or buses. Since you’re going to do 2 attractions within a day. It takes about 30 mins for a cab drive from Jurong bird park to Sentosa island.
You may use this link http://www.worldtaximeter.com/singapore/resort+world+sentosa/jurong+bird+park to estimate the travel time and cab fees for different distances.
Good luck and have a nice trip!