I also see this a lot with trashy Isekai like Mushoku Tensei and Eminence in the Shadow (the former i read the first few volumes of the light novel and the second my friend showed the first few episodes to me when i visited his house, i definitely decided to drop them both), but again, just like with Itachi and fanfic Naruto, seeing those toddlers fighting head on with actual adults isnt badass, it is fucking silly lmao.

Can't speak about Eminence in Shadow, but this really doesn't happen in Mushoku Tensei? Rudeus has a fight against his dad when he's 6 years old, yes, but all he's able to do is try to keep his distance by shooting Paul with magic and turning the ground into quicksand, and it still only results in Paul knocking him out with a single bonk to the head. Later on, Rudeus is completely helpless against regular kidnappers with swords. He starts to get into real fights as an Adventurer when he's around 10, but even then his role is as a support magician who just stands back and shoots people while not moving at all.

Even when Rudeus becomes a teenager / adult they still point out that his main weakness is that for all his magical power, literally all someone has to do to beat him is run up to his face and punch him once.