Overwhelmed w/partners children and ex

Well I have been in Europe for a month now. Things are wonderful with the exception of how much tension there is one we have his children.  He includes me and everything and wants me to be a very much a part of their lives but when I try to offer suggestions or coparent in my own style, he becomes highly sensitive and has stated to me that I am incredibly critical o offer suggestions or coparent in my own style, he becomes highly sensitive and has stated to me that I am incredibly critical of his parenting.
I don't know how to be as involved with his kids and not come across as critical or judgmental. He regularly asked me for my input and help with the kids but I feel like I can't win no matter what I do. The ex-wife is simply awful to him. She sends me nasty messages about what an awful father he is and how he doesn't want to spend time with his kids anymore. This came about  because she change the time that we were to pick up the kids because something came up at her schedule that was not convenient for her, mind you we drive an hour and a half to pick them up, and he said well I will have to pick them up at another time and she said that wouldn't work for her and then she laid in on him about how awful he was as a parent because he didn't have time for his children. I know better than to get involved in that relationship that he has with his ex but she is  pulling all of the strings and manipulating to the extreme. She knows he will never do anything to hurt or harm the children, so she absolutely controls every single thing we do so that it fits around her schedule.

I'm frustrated and overwhelmed and starting to feel a bit resentful. I need time for myself, have expressed this to him but he feels I find the children as an inconvenience.... I have none of my own. We only have them part time but I was not ready for a ready made family and now as I'm trying my best, I'm "critical and too involved" AFTER he's asked me to be involved☹️😞

Honestly?  You've got some serious red flags waving right in front of you.  His kids are always going to be his number one priority (as they should be and it sounds like you want).  I'd think long and hard if this is the life you want to sign up for.  He's a package deal.  It's not just him.  You have to want and be happy with the life you have with him, his children, and the ex-wife.  You and the ex certainly don't need to be BFF's, but she's ALWAYS going to be around.  Even after the kids are adults.

Usually life while you are dating "is as good as it's going to get" as everyone is more or less on their best behaviour.

Big hugs.  I hope you two can find a resolution that works for you both.  Sounds very tough.

I can't even tell you how much I appreciate your input. Thanks so so so much for taking time to respond

Just my 2p.  I missed a big red flag in my first marriage.  My ex really didn't have any friends.  He had friends throughout his life but at some point, everyone had done something in his eyes that was unforgiveable and he just never spoke to them again.  In my very first counselling session after he left, my therapist said, "So it was your turn."  Damn, it was too easy!  It was right there in front of me.

Every relationship is going to have some warning signs.  It's whether it's part of the "I can accept this" or not.  My now-husband is TERRIBLE at making decisions or plans.  While this is the complete opposite of me, I can accept this about him.  I've learned to be more laid back and go with the flow.  And he's a bit better about having a plan.  A teeny teeny tiny bit better.  Granted we have a baby due within the next few weeks and we have no name.  And we won't have a name after he's here.  And I imagine for a few weeks after that...  I have a name.  This is the only time I've wished I was single.  I could just name the kid what I want!   

Seems like you really are stepping into a mine field.  I don't think there is anything you can do that is going to please everyone so don't beat yourself up.  IMHO , you need a strategy for dealing with everything and some rules to make everything happen as it should, regarding how you are going to relate to your partner and his kids when they are around. 

I'd be looking for books and possibly even counselling/ relate to help you both decide how it should work.  The kids are looking to you and your partner for clues about your role and if you guys have nailed it down, it will make the kids feel secure. 

If it makes you feel better, I had a stepmother in my life and sometimes it was incredibly rough.  Over many, many hard years we've developed a nice relationship and Still consider ourselves family, long after my dad has passed away.  This is a chance for you to have more love in your life , even if it doesn't feel like that right now.