Is 1-4-3-1 too defensive for U12?

The formation is not automatically defensive, but it can become defensive if the roles are interpreted that way. In a 1-4-3-1, the outside defenders have a big part to play in giving width and supporting attacks. The midfield also needs to get close enough to the striker so that one player is not trying to connect the whole attack alone. If all four defenders are told their first job is to stay in a line, then yes, the shape can feel very cautious for U12s. I’d watch the distances rather than the numbers on the board. When you have the ball, are there players ahead of it, beside it and behind it? Can one of the outside defenders join? You can keep the same formation and coach it more bravely. At 9v9 I’d change the behaviours before I changed the diagram.