I agree that it may well have been a different nanny and kids in the elevator, otherwise the time line just would not fit.
We really have very few facts in this case, and I am afraid we just have to be patient.
It seems logical that she killed Leo while he was sleeping, which would have been easier and quieter.
We need to stick to logic, and the few facts that we have.
Witness statements are often not reliable, as much as we would like them to be...so I never count those as facts.
My opinion only
There would have to be alot of coincidences for the elevator witness to have identified the wrong children. First, the children were of the right age range, with a girl about 6 and a todder boy in a stroller. There are lots of kids in a building of this type, but not that many, and that particular combination is more rare. Most times I see just one kid with a nanny so two kids is something that would stick out to me. NYC apartments are generally too small for multiple kids, so most families with multiple kids move to the suburbs when they have a school age child. Also, usually when I see multiple kids it is with a parent, not a nanny. So I imagine there would be a very small handful - if any at all - of children in that building with that combination of gender and age with a nanny.
Second, the girl mentioned dancing to the elevator witness. That would be very coincidental for another child to have mentioned that, given that Lulu was suppose to have been at dancing class at that time and might have mentioned dancing because maybe she thought she was going dancing. Also, I am not sure if this is true or not, but if in fact the elevator witness actually saw the nanny and two kids exit at the second floor (the same floor as the Krims), or even if they saw them exit at a floor below their own, that too would be very coincidental for the family not to be them. IMO, it has to be them.
Between the two things, right age range and gender, and child mentioning dancing, it would be pretty coincidental. Add to that the fact (possibly?) that you have the right age and gender, with a child mentioning dancing also getting off on the Krims floor, and I think it is pretty clear cut. Question is where was Nanny with kids between 3:15-4:50?
NYC is a very impersonal place and kids in these buildings really do look interchangeable, especially when you mostly see them in the elevators and they are all buddled up in their jackets, and are in strollers, etc. You don't really recognize or even see their faces. So I think it is much easier to recognize a family group by recognizing the adult in that group, rather than the kids themselves. NYC is not the type of place where, even if you see a family everyday, you will say, "Oh those are the Smith kids." Rather, you might recoginze the man or woman with the kids, but you wouldn't be able to pick out which kids go with which parent, because they all look alike. I know personally if I was in this situation, I would only be able to identify the kids after I saw the picture of the parent or nanny and then I could identify the kids indirectly.
Doorman too don't always see people go in the building. Sometimes, they take a break to go to the bathroom and someone else fills in for a few minutes, or they might help pick up the USPS packages, or they might be outside hailing a cab for someone. So while it is the case that the front door is always monitored, sometimes, they switch off responsibilites on that for a few minutes. I am not sure if this building has two doorman or a doorman and a porter or concierge, but often there are 2 people who swipe off duties.
Also the doorman has to be wrong. He had to have seen them at some time and maybe didn't realize it or else he was on a break. Nanny had to have picked Lulu up at school so she had to have entered the building sometime between 3:15-5:30, and doorman, as I understand it, never saw her entered, so if you took the doorman's word literally, she and the kids would not be in the building at all. We know that is not true, as we know Lulu came home after school, so it must have been the case that the doorman just didn't see them. They had to have entered the building sometime after 3:15 and no one saw them in the building except the elevator witness, as I understand. So the fact that the doorman did not see them has absolutely no bearing on whether the elevator witness identified the right family, as some have said that the fact that the doorman did not see them must mean that the elevator witness is wrong. Holding true to the doorman would also mean that they were not in the apartment at all, or that Lulu never came home from school, either of which we know is not correct so we cannot draw conclusions from the doorman.
Sometimes, I think when the doorman is busy, hailing a cab, etc., he just kinda monitors the door to make sure random people off the street are not coming in. So he might have just saw a nanny and kids entering, not knowing which specific nanny and kids they were, but not stopping and questioning, because they looked like they belonged there. A nanny with two little kids does not pose much a security threat, so them entering might have went over his head if he was busy doing other stuff ("oh, just another nanny with kids,I am not going to stop what I am doing to go see exactly which nanny and kids they are because it is not necessary to know"). Also, as I mentioned, kids with nannies look interchangeable, and especially if this doorman was new, as I think he was, he might not have known specific names or faces or perhaps not be able to say definitely whether or not he saw them. He might not know.
