There are threads with details of swimming schools, you should search to get subjective views.
You have a few options:
1. If you move to a
condo, find out who visits to do swimming lessons (Management Office should know)
2. Send them to a pool close by at a instructor lead lesson like this
http://www.swimminglessons.com.sg/ - we use them for my son now he can swim
3. Teach them to float and swim themselves and wait until they get a bit older for stroke development and then send them to a school. We did this with my son and used the above school, my daughter is now swimming around the pool, we are waiting for a bit before we send he to lessons.
It's totally up to you, I enjoyed spending time with my children teaching them how to swim a little and giving them incentives I knew they would react to well in order to do that.