You can choose anytime you like, however, they will check whether they need helpers at the time slot you choose. Usually, they need more helpers during weekend or late night. Once you decide the time slot, you have to commit to go regularly, e.g. for my case, I went every Sat night from 6-9pm. Yeap, as far as I know, they have only one location (中區mid-level). I usually walk up the hill from the MTR station, it takes about 15 min. Once you call them, they will organize a training orientation for you to teach you the rules and how to take care of babies.
They will not accept pregnant women to be the helpers, as the babies/children (they also have a group of 2 month to 2 yrs old children) may have illness (e.g. 德國麻疹) which is very dangerous to pregnancy women.