GOS Messaging to Apple Watch

Got our kid an LTE apple watch se 3. Screen time works good. They can message me or my wife anytime, but nobody else. My question is, since I’m on GOS, is the only option sms between us? Signal doesn’t have an apple watch app, does it? Nice that they can call me, but I would prefer secure text messages. They have Simplex on their ipad but they don’t have that with them often and it doesn’t have LTE.