you would be suprised how many people do know that they came when He was about 2 years old. Alot of people who have a nativity scene in their home, set the wise men farther away to symbolize their delayed arrival. The problem with acuratly displaying it publicy (ie real big), you would have to put them faaaaaar away and then people would really be confused.
One easy solution would be to dispense with the Manger bit altogether and depict Jesus as living in a house instead. That's where the Magi found Jesus--in a house. Another option would be to place the Magi on the other side of the entrance to the church from where the Manger Scene was set up.
But like I've already insinuated: It's not that Catholics can't figure such things out, it's that they just don't give a sh!t. They could care less if some unlearned soul drives away believing that the Magi were on the scene shortly after Mary gave birth to Jesus. It's just one more lie that the Catholics are guilty of promoting.