iPhones use iMessage to send msgs to other iPhones and texts to Android phones. However, sometimes, the iMessage texts do not reach Android phones. This happens due to stale iMessage registration of the Android phone number (the previous owner of that phone number used an iPhone)
If this is happening to you, then please deregister your child’s phone number from iMessage on this Apple website - Deregister and Turn Off iMessage - Apple Support (Scroll down to the section “No longer have your iPhone”)