Ok. Your 124,000 SPG points can be transferred to AA miles or Singapore's KrisFlyer miles or any of SPG's other transfer partners. For every 20,000 SPG points you transfer you get 5,000 bonus miles. So if you transferred 120,000 SPG points to KrisFlyer, you will have 150,000 KrisFlyer miles. Just make sure you break up the transfers in smaller amount (I suggest transferring 40,000 points at one shot and repeat it 2 more times to get the full bonus miles).

So now you have 150,000 airline miles. First create KrisFlyer account.
Then go to https://www.singaporeair.com/planBoo...execution=e1s1
Search LHR to PER one way and make sure check the box "KrisFlyer redemption
Click "search and book" and you will see flight options with miles needed.
It will cost 144,500 miles for 2 in business or 102,000 miles for 3 in economy plus taxes and fees if you book online after 15% discount on the miles needed.

You have to call the airline to add infant as that can't be done online for most of the airlines. I suggested 3 economy as another option which means you are getting a full seat for the baby but you can also choose to just get 2 economy tickets and add the infant. It won't be as comfortable as other options.

They will make sure to assign seats where bassinets can be used, business or economy. It doesn't matter. Business is much better option, in my opinion, as your child can sleep next to you or the bassinet and you have much more room.