My wife had some missed days of fasting while pregnant and then nursing our sons. Can she give fidya/kaffara for those missed days ? she has missed 45 days and shes in her early 30’s with two kids, 5 yrs and 10 months. What do you advise on how we proceed?

First obligation would be for her to make up those days with no kafara because she had a legitimate Islamic reason to break her fast.
Even if several Ramadans are missed because of legitimate reason, all she needs to do is to make it up slowly till she make all the 45 days.
The only time she can resort to paying kafara in replacement of making up the missed days is when her health does not allow her to make up the missed days and she will be considered like a person with a chronic disease who can just pay fidya in replacement of fasting the missed days.