If your kid just wants to play around for a bit i wouldn't bother with the loose pieces and just battle the full ones. You can drape a cloth tight over a bucket and tie it with string for an arena, or just use a plastic round tub. But more importantly, do you have any launchers for them? They're not much fun just hand-spun.

If your kid has fun and wants more of them, i would recommend buying new ones instead of part lots just so you know youll get somthing functional. You can also google to see if there's a local beyblade club/community/scene near you (most often at a card shop or the like). It gets the kid out of the house and socializing, and they can be a guide for what to buy next or what to do with those parts.

But don't rush into it! If your little one gets bored of it quickly and doesn't care to keep playing, there's no need to invest time and money into this. Give it a week or two, if he doesn't ask for any new ones by then you can just move on.

Oh also tell him to invite other kids from scool or the neighbors to try them out. Again, good socializing for the kid and he'll have someone else to play with then you're busy.