It's quite complicated. First of all, any speacial reason why you are considering sending your kids to a local school?
The first thing you may want to consider is which "2nd language" you want your child to study. Chinese, Malay and Tamil are the only 3 offered. If you don't intend to pick any of these, you will need to apply for an exemption.
Next, going by school age, your 10 yr-old should be placed in Primary 4, which is the streaming year. This is the level at which all children in local schools sit for a "streaming exam", after which the "weaker" students are separated from the "average majority". It is a rather stressful thing to put your children through.
There are many more issues to consider, perhaps you can give more details about the children's background?