Is it worth spending miles on a first-class seat for my infant child?

Not really worth it for infant child.

save it for when he's older

Hahaha why not?

you would make you and your traveling party seems a bit like Crazy Rich Asian..

that's the kind of fame you don't always get

thats I'd provided you have tonnes of miles la hahaha​​​

Call the airline and they will allow discounted price for your child if it fulfil certain criteria

If I'm interpreting your question correctly, you're asking if you should pay for your infant's seat with cash or miles. Unlike what others have commented, I don't think that you're asking if you should fly your infant in F or not.

That depends on your value of a mile. But since you only have to pay 10% of the cash fare for your infant, that's usually the cheaper option.