As others have said: they're not making fun of kids. It's not even meant as a funny cartoon - most of the Charlie Hebdo stuff isn't. They're about the major issue of complete indifference among most Europeans about the fate of the people and children fleeing and dying just for a chance of a sane life. I don't know if it's in the news in the US a lot, but it's pretty much daily news here (and it resulted in me avoiding Facebook altogether because of people I always thought off as sane, normal folks posting horrible bullshit - as if those people are doing this just to get a bit of easy money out of us).
It's fine if you think this is too shocking or overshooting the point, and that's a fair standpoint (I think CH crossed the line many times in the past - not this time though). But you should understand that Charlie Hebdo, as weird as it may sound, is not primarily in the business of making you laugh. Definitely when they tackle major human rights issues like this. It's about holding a dark mirror opposite ourselves, rather than 'the other' (in this case - not always). They're fueled by anger about social injustice more than some need to make people laugh.
I'm not trying to come across as a CH apologist because they've published plenty of shit that was more damaging than helpful, but in this case I can't blame them. It's shocking - yes. But it should be. This shit has been going on for way too long over here now, and when pictures and clips of drowned kids washing ashore started to surface people actually thought more of us would actually start to care, and they didn't. This cartoon is mostly about how fucked up that is.
Again, it's not trying to make a particularly deep point. It's just trying to illustrate how fucked up the attitude of a lot of people here (including, I'm sure, part of their readership) is right now. If we allow actually pictures of these kids to be used in debates in newspapers and talkshows (and we do, and I agree with that - the truth must be seen, as fucked up as it is), we should be able to allow cartoons like this. This might just have the effect of some people thinking twice about how they're reacting to all of this. It might not. But we need as many voices as possible speaking out about it. They did the right thing with this cartoon.