Watch the second Spy Kids. There is an scene where the kids are fighting skeletons on the island. The skeletons are the same size as them. As in, undead children!
Or possibly undead midgets.Or pigmys.
While we don't see anyone die in the films, it is indicated that they do go on other missions (i.e., after the first film). They've probably had these other missions go a lot worse, and the whole series could be taken Darker and Edgier and if you think about children being forced to kill its pretty horrifying. No wonder Juni wants to quit at the end of Spy Kids 2.
When we first see the footage of Carmen in the third movie, both of her arms are normal, later we see her with a robotic left arm, if Carmen was plugged in like Juni was, and what I think happened happened (she lost her arm in like the Mega Race or something like that) she felt the pain of her arm getting torn off geez.