Well, I see two really bad flaws in this game. If you hold space bar, you create a "rock wall" and instantly dispatch of a column of boys. There's no time interval between the rocks. So all it takes to win is to hold space bar right at the start of a level and wait for the enemies to crash into it.
And the second problem, the difficulty is increased by adding a row of boys every level. But there's no limit either, so if you reach level 9, you die. Instantly. It's because the grid has grown too big, so right at the start of the level, enemies touch you.
As for other things, there is no sound and the graphics are too simplistic (rocks have a black background, boys' sprites have a green border, the white lines behind the rocks are unnecessary and at the end of the level, rocks fly above the "Level Up" button, not below it). Also, screen boundaries are a little off, so both you and the boys can move off the right edge of the screen, but you can't reach the left edge.
Then I could probably say there's a little variety, e.g. there could have been more than one kind of enemy, power-ups, anything. But shooting the target at the top every once a while is a nice idea.
If you are a beginner, don't worry and keep learning, perhaps with a different tutorial. Everybody must create a hefty amount of inferior games before being able to create the great ones. Good luck.