I think it’s a strong possibility the daughter was with her when she put Lucas somewhere so focusing on a place where she can still see her car is important.
Although this is valid, i also think it more likely that she left the baby home sleeping. We are focusing on night time hours. They rent a house, not an apartment. Even if she did wake up, who is going to hear here fussing? She also may have been accustomed to "putting down" the kids (terminology she used in her interview) with help from a med when needed.
Also, someone asked earlier why would she do marijuana in the garage and not inside. If a CPS investigation was imminent due to the ex husb charge against her boyfriend, she probably knew they would be looking throughout the house only. They don't search places where the kids wouldn't normally be.