<human>: Can you condense the text into a few key points?

I am currently dating a woman who just moved across town, she is currently living with a guy she's been friends with for a long time. We've been dating for 8 months and I've met him multiple time and have never had an issue with him. He has been respectful and she has assured me he has absolutely no intention of trying anything. 

Here is my issue. She has two children, ages 12 & 7, who stayed with their dad and at her house while she was gone. Her and I discussed me stopping by this week and seeing them. When I tried to coordinate this with her roommate I told him I wanted to come over and make "welcome home" signs with the kids for their mom. He said it wasn't a good time and wouldn't work out so I respected this and was disappointed. She got home today and I found out that on Friday her roommate went out and got them poster boards and told them to make the signs. I'm pretty livid about it since it was something sweet I wanted to do for her and something that would allow me to continue building a relationship with her kids. He never mentioned to her that it was my idea, so he looks like a great friend and I feel like I got shit on.

So here's my question, do I say anything to her? If so how do I bring it up?
<bot>: GF went on a trip, I wanted to make her welcome home signs with her kids, her roommate stole this idea and had the kids do it without giving any credit. How do I handle this tactfully?