We are talking about Singapore CPF system not some other country's.

Below are from the CPF website:

"The CPF Minimum Sum (MS) Topping-Up Scheme allows you to give top-ups to yourself and/or your loved ones’ Special Accounts (for recipients below age 55) or Retirement Accounts (for recipients age 55 and above). Such gifts help build up the MS and can come from your CPF or in cash.

To make a top-up using your CPF for your loved ones*, the net balances in your Ordinary and Special Accounts, including amount withdrawn for investments, must be more than the prevailing Minimum Sum. Only Ordinary Account balances can be used for the top-up.

* "Loved ones" include only your parents, parents-in-law, grandparents, grandparents-in-law, spouse and siblings."

Just like they told me, topping up for children's CPF not allowed.