So you should do a voluntary top up to all three accounts.
I tend to disagree, with a key assumption (below).
A child in good health has the longest possible time horizon among savers. Given that, the prudent course is for the child to take a somewhat more aggressive investment posture than older savers(*) -- using vehicles that have short-term risk but higher long-term yields (with reasonable probability). That's not CPF OA
if you know what you're doing. (Many people don't know what they're doing, of course. In fairness, it's somewhat tougher to do better than CPF OA onshore in Singapore, particularly at CPF bonus interest rates.) CPF SA and MA are
closer to that more aggressive investment stance. With equities markets rather high right now I could see CPF (MA and/or SA) as a reasonable savings vehicle for the higher yield. Not for
every dollar, but for some. MA can also pay for medical insurance, MediShield Life premiums, and maternity bills (for healthy grandchildren!) if nothing else, so why not toss a few dollars in that direction?
(*) With a couple caveats: saving for future tuition and housing down payments. Which means you probably ought to have a couple different "buckets" of savings for that child, with different allocations. The "college/young adult" bucket has roughly a 20 year time horizon from birth, while the rest can run much longer.