There’s one present in every level except for Build A Star 1 and Make Cancer, according to the folks at GameFAQs. I found the ones in each Build A Star level, Build The Moon and Make The North Star.
The presents in BAStar 3 and (I think) BAStar 9 are total bitches to find because they actually aren’t just scattered in any random location. I had to get help to find both - in particular there was no way I would have ever found 9’s without knowing exactly what to do. For the others, I suggest not worrying about them until after you win the level once. Replay the level, and only worry about getting big enough to get past any of the “must be over 5M or whatever” barriers. Otherwise, spend a lot of time finding a safe vantage point and looking around. Almost all of them are found in mid-air, in some form or fashion.
As far as Make A Star 7 goes, most people seem to get stuck here because they miss out on a major part of the level that’s not necessarily easy to find. Go in the schoolyard, and after picking up all the stuff there, look at the wall over which you first climbed to enter the schoolyard. On the far right end (that is, when you look at it from the inside of the yard), you’ll see a hole. In MAStar 6, this hole in the wall only leads you to the roof of the gas station where you can pick up a few more items - no big deal. In MAStar 7, it leads you to a brand new section of the city. I think you need to be 3m to bypass the barrier located there.
Be sure to not only explore the new part of town but to go up the hill nearby - there’s a cow farm there, and by the time you’re big enough to get there, you’ll be big enough to pick up both the cows and the fences.
Once you’re done there, go back to the first part of town and maybe by the beach - the only thing that should stand in your way now are the trees and houses. Everything else will be fair game. It should be easy to win at that point.