Does you Granddaughter have an iPhone - or just an iPad? If exchanging messages between Android and Apple devices, you’ll need to use SMS/MMS messaging. With an iPhone this is easy - however, an iPad without the aid of an iPhone cannot send or receive SMS/MMS messages.
In more detail - an iPad, whether a Cellular model or not, is not capable of directly accessing Cellular Voice or SMS/MMS Messaging features. An iPad with either WiFi or Cellular connectivity alone can only access IP-data services. When adding a DataSIM and Cellular plan to a WiFi+Cellular model of iPad, the number associated with the SIM card is only used to identify the associated account for billing purposes. This number is not used for making calls or sending messages.
However, when paired with an iPhone, the capabilities of iPad significantly broaden to include access to Cellular Voice and Messaging services of the associated iPhone - relayed to the iPad over WiFi using Apple’s continuity features.
More information about Continuity and the relevant Cellular services can be found here:
Continuity
Use Continuity to connect your Mac, iPhone, iPad, iPod touch and Apple Watch – Apple Support
SMS/MMS Messaging
How to forward SMS/MMS text messages from your iPhone to your iPad, iPod touch or Mac - Apple Support
If your Granddaughter has an iPhone - and both iPad and iPhone are signed-in to iCloud with the same AppleID - to send SMS/MMS messages (green bubbles) from her iPad she must explicitly enable Text Message Forwarding for the iPad on her iPhone...
On iPhone:
Settings > Messages > Text Message Forwarding - set to ON - and explicitly enable the iPad from the list of devices.
Also ensure that Messages in iCloud is enabled on both your iPhone and iPad:
Settings > [Your Name / AppleID] > Messages - set to ON