A Day at Yokohama

We're planning to do a side trip to Yokohama this May with our 11m baby. I'll appreciate advise on

1. Is half day sufficient or a whole day better?
2. Other than Chinatown, what are the must-see sights?
3. What's the best time to go to Chinatown- morning or night?

t.i.a.

1. Is half day sufficient or a whole day better?

Depends on how much you want to see. Half a day will allow you to see only one or two sights.

Other than Chinatown, what are the must-see sights?

Yokohama does not really have any "must-see" places. I would just visit the sights that look interesting to you:
http://www.japan-guide.com/e/e2156.html

What's the best time to go to Chinatown- morning or night?

Lunch or dinner time or in between.

Please refer to the link for a famous spot in Yokohama.
The most famous sightseeing spots are Chinatown area, Yamashita Park area, and
Minatomirai area. It is possible to move on foot between these spots.
However, it is insufficient to go sightseeing slowly in these three spots at half a day.
For your information, the course when I went to Yokohama is as follows.
1. Chinatown Area¥¥¥sightseeing and lunch
« ( walk ; about 3 minutes )
2. Yamashita Park Area¥¥¥sightseeing
« ( walk ; about 5 minutes )
3. Minato Mirai 21 Area¥¥¥sightseeing and dinner
In addition, Motomachi Yamate area is also recommended.
http://www.welcome.city.yokohama.jp/eng/tourism/index.html