The only reasonable thing I can think of, and I don't know the details so this could be all wrong, is that the cops thought the situation had become a hostage / barricaded suspect.

It sounds like the shooter didn't spend much time roaming the school and went into one specific classroom where he did the majority of the killing, much like Sandy Hook. If this is the case, someone may have decided to treat the situation like he was holding children hostage in the room, unsure whether or not the shooter or the children were alive. This would require the area to be sealed and the other classrooms to be cleared and removed one by one when it was able to be.

If there was a hostage situation, it would require a very skilled entry team with the right equipment to make entry into the room, which would easily take an hour, just to drive in all of the needed equipment and personnel to the right place, get layouts of the building, and plan an assault (which will still be extremely risky). Having individual beat cops go charging into that room that the suspect may be barricaded in basically guarantees that they, the shooter, and the children will most likely die from the resulting gun battle. That's why you would instead want to wait to use specialists.

Now, this is different if the police hear shooting in the room, at that point, the gun battle would result in less children being dead then waiting. The question now becomes, why didn't the cops know that when they got on scene, that the children were being executed. Could be that the shooter shot them all initially, or it could be they just didn't know from the initial confusion on the scene.

Now about other officers rescuing their kids. That's going to happen, you can't stop it. The French Army (due to previous military failures) has actually learned this lesson the hard way several times. In World War 1, World War 2, and the Franco-Prussian War, French military units started to loose good order and discipline any time troops started to approach their own home towns. They would repeatedly abandon their posts, whether engaged in battle or not, in an attempt to help their families evacuate. The only way to prevent this is to basically make sure none of your military units are comprised of single geographic locations, that the locations being attacked are not the same ones as where the soldiers are form, and to ensure the soldiers are aware that evacuations are already underway. I'm not surprised the cops did that. And if you're mad that they still enforced the barricade, see the above portion about a hostage situation. The cops are always going to rescue their children even if it's against policy, even if it's a bad idea, even if it puts other people in danger, even if they tell others not to do it. It's a human problem more than a procedural one. Other police (who don't have kids inside) are going to stop families from stepping in, probably because it is a good idea to stop them. Considering the length of time this is taking place in, losing control of the barricade is going to turn the situation from an active shooting, to a riot, to an active shooting during a riot.