This crap about women & children being left to get shot while fit young men storm across Europe conducting some kind of jihad by stealth is damaging and conceals the real problems, which are many and severe.
Getting logistics out of the way first - leaving Syria, Afghanistan and Libya for unknown destinations in Europe is physically gruelling, very dangerous and very expensive. The refugees Europe's seeing at the moment are those who had both the money and enough strength to tackle it. Thousands of times more refugees, mostly families, are slowly starving in Lebanon which is geographically closer and presents fewer issues with language. International aid to the enormous camps in Lebanon has recently dried up; it was already inadequate. There is little hope for the 2 million people there now. Lebanon stopped registering new refugees several months ago, but they are still arriving.
Rape is an horrific problem in Syria, as it has proved to be in many other wars. Women and children are routinely raped, usually at gunpoint and usually with extreme violence. I would have linked to some reports but they're really gruesome. A high proportion of victims die of the injuries, or are simply killed as part of the show. Many child survivors will have been raped & tortured themselves; they are very likely to have seen their relatives or siblings abused like this. The young men arriving have seen this happening to their wives, sisters, mothers, children, friends. It's unlikely that any refugee has an intact family; they have all seen people they love killed in violence. They are traumatised. Also, as intended, they are ashamed by the rapes. And ashamed of their shame.
The trauma of living through intensely awful events, one after the other and continuously for years, then trudging through foreign lands with your remaining possessions in a backpack, to be further abused and endangered, doesn't leave many people unchanged. Our responses to trauma are unpredictable. The level of care needed to enable a safe recovery isn't available - it should be, but we don't even bother to provide it for our own deeply distressed people let alone an influx of more.
Displaced people have always been viewed with suspicion, and this is largely the reason. It is a real problem: not the fiction of terrorists putting themselves through that just to attack us (why? we've got plenty of home-grown terrorists, and IS ain't short of a bob or two to send agents over here on normal airlines.) But the reality of terribly hurt, shocked, frightened, angry, anxious, bereaved & bereft people landing up in our societies with no clear pathways to recovery and settlement.
We could help with this, we really could. We won't, because we're tight and petty-minded. The skills and the means exist. Just not the will. Because displaced people will not be helped as fully and compassionately as they could be, they will be displaced - traumatised, unpredictable, lost - as they've been throughout history when such things happen. As PPs have pointed out, we never seem to learn.