There is no one-size answer to whether kids should watch UFC. It depends on the age, the kid, and how you frame what they are seeing.
For kids under eight, the answer is generally no. Not because violence in media necessarily creates violent children — the research on that is more complicated than most people realize — but because MMA at its most intense involves bloodied fighters continuing through cuts, and that is genuinely disturbing imagery for young children who do not have the emotional framework to process it yet.
Ages eight to eleven is where it gets interesting. Kids this age can watch the sport if it is framed correctly. What is the sport? Two trained athletes competing under a rule system to determine who is better at a set of skills. What is the goal? To win by proving your technique is superior, not to hurt the person. The distinction between controlled competition and actual violence is something kids this age can understand if an adult helps them understand it.
Twelve and up, in most cases, can watch with minimal mediation. They understand competition, they can contextualize the violence within a sporting frame, and many of them are already interested in combat sports or practicing them.
Practical tips: watch together and narrate. Point out the technique. Name what the fighters are doing — that is a double leg takedown, that is a rear naked choke, that is a head movement slip. The sport becomes much less viscerally disturbing when you understand that what looks like chaos is actually highly structured trained movement.
And if your kid watches one grappling exchange and says they want to try jiu-jitsu, take them. Seriously.