The bolt is closed?...... Does the rifle feed/perform flawlessly? Some wiggle is normal.....and necessary. You notice airguns for the most part do not employ any type of magazine lock like a standard firearm, most of them are designed to "float" slightly but not fall out under normal usage. While the bolt is closed the mag can't go anywhere anyhaw, it's only during the the moment the bolt/probe is retracted beyond the face of the mag that it can be removed. I believe this is part of the manufacturing process and allows for a wider tolerance amongst guns and mag's being interchangeable without physical alteration. Just take a look at ar15/Ar10 platform guns for instance.....the mag slops around quite a bit, even on $5k models. As with all things firearms......SIMPLE-ROBUST, AND SIMPLE are the finest qualities you can impart during the design stages. It should work for your grandson as well as it worked for you......and never need a single part replaced. Ahhhh the good old days..
P.s. is that a factory fx mag?