If those are your two choices get the 50mm ƒ/2.
I used the 60mm ƒ/2.4 on an X-Pro1 for years. While generally speaking the autofocus isn't as bad as people make it out to be, under low interior lighting, the autofocus struggles and struggles hard. Especially when trying to lock on to human faces such as eyes, indoors the camera will struggle to get enough light to detect contrast. You will end up with a lot of back focused pictures. Or you may try to take the shot only to find the lens spends 10 seconds hunting before giving up. You can manually focus, which is what I often did but with the dim and low-rez EVF of the X-Pro1 it isn't much fun. The lens is far better paired with newer sensors such as that of the X-Pro3, where it really comes into its own due to the advanced phase and contrast detection, better EVF panel and focus limiter. That said, I've never regretted getting it over the 56 ƒ/1.2 or any of the new short telephoto lenses that followed. I've made many pictures I'm proud of with it, especially paired with the X-Pro1. The combo is certainly doable, but your use case runs up against every single weakness that lens has. If you're relying on it to get once-in-a-lifetime pictures of your grandson, you will regret it.
To be clear, I've never used the 50mm ƒ/2. But I have used the 23mm ƒ/2 and 35mm ƒ/2 Fuji lenses on my X-Pro1. The performance improvement over the original lenses is significant, and the haptics so much more solid. Tests I've seen show the 50mm is the sharpest of the three. Buy with confidence.
