The initial limited English only blacklists were useless. Kids are creative and terrible. So we replaced with big processes of failover with several external swearword filtering services. Costly...
But kids are creative..., they write single letter messages that circumvent any automated filtering. Or rearranged their "room" with decoration that spelt out letters, or simply drew cock and balls, etc. This font might have reduced the need for filtering. But kids are creative...
Thankfully it was not instant messaging. Direct near-instant messaging came in later products... Car-crash every time. Never write a messaging app for kids. There is always a mean-time-to-dickpick or worse.