Can I transfer iTunes balance to my grandson phone

No, your balance is tied to your account, you can't transfer any part of it to another account (a balance is tied to an account, not a device) nor can you use it to gift content (it can only be used to buy content for your account)

If you've only just redeemed the gift card onto your account and not spent any of it you could try contacting iTunes Support and see if they can/will transfer the card's amount to your grandson's account (or 'unredeem' the card)