My 20mo has started twitching when he goes to sleep at night, I’ve noticed it when he naps in the day or overnight (he ends up in our bed from midnight-ish so I’d feel it).
When he first goes to sleep his arms/legs/hands just randomly twitch, not massively so, if he was under a duvet you probably wouldn’t see it but he’s next to me so I can feel it. It’s almost like when you see a dog sleep and their paws move, it’s sort of like that. It’s definitely not constant, just random for maybe the first 5-10 mins when he goes to sleep. I’m scared to Google.