409-220-5313

This person keeps sending sexual messages to my ten year old granddaughter's emergency cell phone and I don't know how to block it.  I ask her to stop and told her she had the wrong number but she keeps sending them anyway.

It's a spammer, phishing for active phone numbers.

Congratulations, by texting back, you only proved to the spammer that the phone number is valid, has value to other spammers, and they and others will started sending more spam and scam messages.

Do not reply, message back STOP, or visit any link or call any number in the text messages.
Forward the text messages to SPAM (7726), this will notify your carrier that the message is spam and they will take measures to prevent the sender from future spams.
https://www.fcc.gov/guides/spam-unwanted-text-messages-and-email

If your cell phone is a smart phone, there are many apps to filter and block spam and scam messages and calls, check you phones app store.
If its a regular cell phone, contact your carrier and request that "premium text messages" and "web based text messages" also known as "text messages not sent from a cell phone" be blocked from your service. If, as you said, it is just a 911/Emergency cell phone, you may want to have your carrier completely disable text messages.