How much should I set aside monthly for my child (18 months old) and where would you put the money?

It depends on the plans that you have for him and your personal circumstances.

Do you have enough money for your own expenses?

Is your son healthy (i.e. do you need to set aside money for medical expenses)?

Is he inclined to and academically able enough to go to university?

Once you've set out your answer to these and other relevant questions, you could do an online search to estimate the cost of each of these considerations

It's first and most important to max out the CDA, 1st child just max out $9k, including government's dollar to dollar matching.

Certainly, I think it's also worth investing for child's future education, possible avenues could be STI ETFs, SSB, or even endowment policies.

I will set aside at least $200 a mth for him

This is not just for children education, but more of a big lump sum for him to use later. In case my children

1. need to go overseas for university (local uni are cheaper by a lot)

2. business to start?

3. or for his marriage and his other family commitments

I will say save 5 to 10% of your income. Save more if you can afford it, but keep it towards your own retirement.

If you can take care of yourself in the future, that would be a great gift for your children.​​​

You should max out the CDA for your child. Think about how much education will cost and divide it by the number of years to reach there. You could put it in a high interest savings account if you're not investing it.