It really depends on whether it is a sunny day. Just come back last night. Basically not really need a down jacket for adult, but would be best for a child. A gore-tex jacket will be fine for adults, or just a thick fleece jacket would also be fine for children. Don't wear too much inside and "bottle neck" clothes inside as you will find it extremely hot when you get indoor. Take a scarf with you. When sunshine gone in sunset time, you will find it a lot cooler.
Disney is a bit cooler than outside, especially during night time. And make sure you take enough clothes inside if you decide to stay until the fireworks. You may leave your thick coat in the lockers inside during the day time.
4月2日應該可以賞櫻 la, check the weather before you leave HK as they are quite accurate, or else you could only see umbrella and not sakura. I went to Ueno on 28/3 and could only find 5-10% sakura opened already.