thinks, "matches (are) dangerous, but cards (are) fatal".  She means that to be caught gambling at all would invite punishment, but to be caught actually using cards would bring far more severe consequences.  To be playing with matches, even when an element of gambling is involved, indicates a certain naivete and lack of sophistication , but to be gambling with actual cards is the real thing.

The children are in this predicament because Jem and Dill had decided to trespass at the Radley place to look in the window in hopes of catching a glimpse of the infamous Boo.  Unfortunately for them, Mr. Nathan Radley saw them, and, not recognizing them as children, let off a shotgun blast to scare them off.  When the young interlopers tried to flee, Jem caught his pants on the fence, and wriggled out of them to escape.  When he, Scout, and Dill were confronted by the neighbors, Jem was embarrassingly pants-less, and had to quickly come up with an excuse for his situation.  It is then that Dill offered that Jem lost his pants playing strip poker (Chapter 6).

In chapter six, the children raid the Radley yard at night and narrowly avoid getting shot after Nathan Radley runs out of his house wielding a shotgun. As the children are fleeing the yard, Jem gets his pants caught underneath the fence and is forced to leave them behind in order to escape. After running across the schoolyard and catching their breath in their back yard, Jem suggests that they walk down to where the group of neighbors has gathered outside of the Radley home.

When the children join the group of concerned neighbors, Atticus notices that Jem isn't wearing any pants. When he asks Jem what happened to his pants, Dill comes up with a lie, telling Atticus that he won them from Jem. Dill proceeds to tell the adults they were playing strip-poker. When Atticus asks if they were playing with cards, Jem says they were playing with matches and Scout mentions,

I admired my brother. Matches were dangerous, but cards were fatal. (Lee, 56)

Scout understands that they risk being punished for gambling but believes that their punishment will be less severe if they admit to betting with matches instead of cards.

Technically, Scout thinks that there is more harm to be had, in the form of punishment that is, in playing cards than in playing with matches. Two factors inform her opinion: she's very young and has probably never been in trouble for setting a fire, but has been in trouble for "gambling;" they live in the South, where gambling is looked down on as a serious sin, especially by Baptists. 

In Ch. 6 Jem comes home with no pants on because he got them caught on the Radley fence as he ran out of their yard (after sneaking in). He cannot say that, however, as he would get in big trouble, so he lies and says that he, Scout, and Dill were playing strip poker. To this Atticus asks if they were playing cards, and Jem says that they were only playing with matches. Thus, Scout says, "Matches were dangerous, but cards were fatal." While this might sound like a strange comparison to most readers, her points of comparison are fairly narrow and she has yet to experience the trouble that matches can cause.