You can just buy your infant their own seat like you would any other older child. Might as well sign them up for Rapid Rewards while you are at it and start collecting points.

Personally I never buy my infant a seat on WN because 50% of the time, if the plane is not full, we just take an empty/extra middle seat for them anyways.

I don't see why the flight attendant would insist they "sit" in their seat though. Just explain to the FAs as they come around doing preflight checks that your child has a confirmed seat (so that they do not mistakenly miscount them as a lap child) and everything will be ok. No need to bring on the child seat if you don't want to.