The image is a little under exposed, so the some of the joints are harder to visualize. There is fusion in the middle joints, but not completely. There are still some open joints in the middle. Some active bone on the front. Looks like a small spur along the front not as bad as it could be, but something to watch. Overall it's not a gnarly joint, but it's active. Might be worth investigating why 5yr old has such one sided arthritis.
Complete fusion is better than partial. Once the middle and lower joints are fully fused there is no movement and no pain. The upper joint is the only one that really need to move. You might get a small amount of decreased RoM, but its better than being gimpy.
Have you looked into more aggressive joint care? IA injections while there is room, IM injection to support and extend them.