The murders appear planned, and with a sexual design.
The sexual nature of the crime seems evident by the adults being found nude, as well as the semen stain found on the mattress between the woman's legs. While possibly it is her husband's, more probably it was left by the assailant.
That the crime was planned is evident by the findings of duct tape as well as the cable ties. As there are no signs of struggle, he (I am assuming the perpetrator to be male) probably brought with him a gun. I imagine he would have had one of the adults bind the other while holding them at gunpoint, and he would have then bound them together, although it is possible he did all the binding himself.
That he entered through an open window and brought tools with him, suggests that he may have been familiar with the house. It is possible he had recently done some service work in the home, or that he was acquainted with the family, but both of those scenarios are unlikely if he did indeed leave his own semen at the scene. It seems most probable to me that the man selected out the woman as a target, and he began to stalk near her home to pattern the family's behavior, as well as gain knowledge of potential points of entry.
The order of events does not seem like it can be entirely deduced from the given evidence, nor can we we have an idea of how well he may have known the layout of the home from given information. Did he find the children or the adults first? If he found the children, was he able to not stir them from sleep to find the adults? Were the adults having sex at the time, and so he was easily able to find them? Or did he by chance find them first, and then have them disrobed and bound? This all seems unclear.
The lack of struggle makes it seem that either a) the perpetrator found the adults first, or b) he found the children first and brought one or all with him held hostage. At any rate he was able to collect everyone in one room and have them bound. The lack of marks on the children indicate that they were either coerced (perhaps by the same instrument that left the lacerations on the adults), or they were beckoned to follow what the man told them to do by their parents. Likely the parents were first bound, then the children.
That the man and woman were found severely disfigured hints at a sadistic element to the crime. So what we are looking at here is a planned, sadistic, and probably sexually-motivated crime. The fact that the children were killed as well, and bound to the foot of the bed of their parents, also hints at a sadistic component to the crime. All of this alludes that the perpetrator is likely a male with a psychopathic personality, and potentially a serial killer, or is on his way to becoming one.
This is what I imagine happened: The perpetrator scoped out the house, and eventually one night (the children were in sleeping garments), went in and had the family bound in the fashion police discovered them in.
He suffocated the children in front of their parents, or possibly after killing them. Whatever order things happened in, the family was brought together so that they could experience each other suffering. I imagine it was the parents who were meant to suffer the worst (they had laceration marks, the children appeared unmarked), so their children were probably killed first so the parents could see that happen.
The parents were bound together and lacerated together so they could "share the experience." I imagine the woman was raped before the couple died from their wounds, most likely before there were any lacerations, and possibly before the children were killed, so that they could hear it. The children may have been left alive to hear their parents up until the perpetrator decided to kill the parents, at which point he would have killed the children in front of them. The mutilations were inflicted in such an order to cause them excruciating pain before the assailant decided to cut their throats.