To ask the question, "Why didn't Obama fix it" or "Why weren't Democrats critical?" ignores two important points that must be taken together:
1)Obama wasn't a dictator (even though trump supporters clearly believe that this is precisely the role of the Presidency), so he couldn't solve everything he would have liked to. In addition, the notion that if he didn't fix everything then he fixed nothing is even dumber than the Nirvana Fallacy.
2)Considering his stance on humanitarian principles, it's fair to say we would have fixed it if he had the political momentum on his side. To whit:
He worked to help the environment, which helps people in poor countries who are most at risk from environmental catastrophes, he increased aid to Syrian refugees, worked to end the ebola epidemic, brought in the masses of child refugees that were sweeping across the border, created the Dreamer Act, and spent most of his first term bringing health care to people who couldn't afford it. And of course he wanted to enact common sense gun control policies but was thoroughly stymied by Congress.
So to make the claim that Obama didn't care about the children at the border is at direct odds with his words and the policies he fought for and was able to succeed in implementing. Nearly everything in his Presidency is about moving toward humanitarian principles rather than away from them like with the current administration.
But let's be fair here: this topic is intended solely as a cudgel to (fallaciously) beat Democrats with. Republicans don't even think the people at the border are human, so all if this is really quite funny to them. None of which should be surprising, because the Trump movement is a White Nationalist movement, and if you want these assholes out of power then you're going to have to vote them out of power.