No. I’d gamble kids tell their parents less than 25% of what parents wish they knew about their kids. Some of the breach could be gapped though if parents didn’t feel like such politically correct intrusive wussies and would have direct question/answer/discussions with their kids. Put on the spot, most kids will tell you what’s up, in a roundabout way, seeds of truth by omission and that sort but if you pay attention, you’ll get it.
I have a very well adjusted ex stepdaughter who at first was very insular but I tried to create a system she could recognize to share with her dad and I. My fiancee now has three kids which are very insular and he and I are talking together how to make routines in our home so when the kids visit, we can feel updated and maybe on occasion, useful/helpful.
Being a step parent is rough! You don’t want to act like you know the kids right off or that they should trust or like you but eventually it is your role to aid in parenting role modeling, etc. Sometimes it’s like we plan strategy more than humor the kids but we do it in order for them to go out into the world safer, more aware, more confident, feeling more supported.