First, almost all US tournaments allow playing up, unless specified in announcements. You can safely assume that, and kindly request to play up.
Second, some parents may think the rating their children get after playing up may be artificially higher than their real strength. It's definitely possible but no need to worry. Playing chess depends not only on one's knowledge and skill, but also on his/her confidence. The higher the rating, the more confidence s/he will feel. It's positive feedback. With higher rating, also comes higher requirements, and your child will improve faster.
A few examples from last Labor Day tournaments.
Student A: having stuck at 500 level for a while. His parent didn't know he could play up. This time confirmed with me, he signed up for U900 section (not U600), ranking at the bottom of the section. Played well, he tied for the second and his rating jumped by 320.
Student B: around 900. I suggested to play in U1200, but she wanted a trophy. Concurred by another coach, she signed up U900 as 2nd seed. But didn't play well at all, losing first two rounds. Of course she didn't get trophy, and was greatly disappointed.
Student C: also didn't perform great in the tournament, only got 2.5 out of 6. But because he was playing up, he didn't lose any points, instead gaining 40 points.