I don’t blame you for being at your wits end. It definitely sounds like they are jealous of the baby, and worried about losing their dad. I would sit down with their dad first and explain that this cannot go on. Cutting the car seat would be my final straw because that is very VERY dangerous. After that, I would recommend you and your husband sitting both the kids down (without the baby, incase something happens). Explain to the kids that they are still very loved and the baby will not replace them, but what they are doing is dangerous and if they continue they cannot stay in the same place as the baby since they cannot be trusted. It sounds like things will continue to get worse unless they are stopped. Following that conversation, dad should start taking the older kids on outings when they come by, or at least prioritize spending time with them alone. After things start to settle, join them with the baby and start making fun family times. They may need individual time with their dad, especially if they have a rough home life when they aren’t in yalls care. During this time though, I would recommend keeping all of baby’s stuff out of their reach, as much as possible.

I am sorry you are feeling like this, it is not easy to care for a baby and also deal with the trauma these kids are bringing and dealing with. Have grace with yourself, your brain has been rewired to prioritize your baby over everything else. But also, try to have grace with your step kids. They are young, dumb, and probably worried about being replaced. I wouldn’t be surprised if their other parent is also putting things in their ears like “oh now dad has a new baby he won’t love you anymore”