We have a couple little ones that have done this in the past, for example a Mini 2 to a Youth 3, Mini 1 to Youth 2 etc. The hardest part I notice isn't the extra gym time but the competitions, which can be very taxing especially if they teams are in the same level like a Mini 2 to Youth 2 situation. You have the child performing with the first team and literally running off stage back to warm ups, chugging some water or Gatorade and going right back in. It can be exhausting.
In level 2 to 3 crossing you often run into a situation where the levels 1 and 2 teams compete in the morning, have awards and then the level 3s compete later at night, meaning you'll be in a convention center for a loooong time. Some kids don't handle this well, especially when friends from team one get to go to the beach or the pool in the afternoon and they have to stay. You also run into awards overlapping with warm ups, so you might have to miss your first team's awards to warm up with the second.
These are just a few things I notice kids struggling with the most