I suggest you to go to Osaka Castle, nice walking from the park and you can take a train inside the park. only 100yen/person.
for universal studio, you can go to Ocean Park website, there is a coupon that you can print and you can buy the tickets cheaper at universal studio. But there is height limit for child. so you better check if your kid is tall enough... otherwise, it is quite boring for ur kid to wait while you are playing... and I suggest you to buy the pastpass cos the wait is like 60-70mins for famous games like spiderman, rollercoaster. Also I suggest you to bring lots of water, sandwich/food inside. cos food inside are very expensive and taste horrible! There is a convenience store right outside the trainstation (universal city station), you can buy it from there.