Blocking text only from ipad but not iphone?

Your AppleID Password should be known to you - and you only. If you suspect that your AppleID account or associated Password are known to others, change your AppleID Password immediately:

If you think your Apple ID has been compromised - Apple Support

If anyone other than you can access your AppleID account, there is a very real risk that you may lose control of the account - and be permanently locked-out with no means of recovery.

Your post suggests that the iPad used by your Grandson is using your AppleID. Sharing an AppleID between users is a bad idea. Outside of Enterprise and Educational environments supporting multiple users, an iPad is essentially a single-user device.

Your child should have their own AppleID - in fact, every user should have their own AppleID. This is essential to maintain intended separation of activities and to avoid unintended sharing of unintended information and data. Done properly, you’ll not have any issues with content related to your child appearing on your own devices - or vice versa.

You may find this information to be helpful with setting-up an AppleID for a minor - and in considering appropriate controls while continuing to share Apps and paid Apple services if you should choose to do so:

Apple for Kids - Official Apple Support

Help your child set up an iPhone, iPad, or iPod touch - Apple Support

Family Sharing and Apple ID for your child – Apple Support

Use parental controls on your child's iPhone, iPad and iPod touch – Apple Support