Can i use an old iphone for wifi only

I believe it should be. I bought my iPhone unlocked, and used it as an "iPod touch" for a few days until my carrier provided a SIM card to activate its cellular capabilities.

Keep in mind that an old phone might not be able to access all Wi-Fi networks and all Web sites. If the phone is very old, there might be only a limited selection of apps available in the App Store for it – if it can access the App Store at all.

What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

If your grandchild wants to use the iPhone as a music player (for playing music purchased on a CD, or online), he, she, or his/her parents may want to refer to:

Sync your iPhone, iPad, or iPod using your computer - Apple Support