My [33M] GF [29F] of 7 months told my son [3M] that we're getting married. We're not engaged.

Here's some backstory: We've been dating for nearly 7 months and the pacing of things has really been thrown off by coronavirus. I live with my brother [28M] and have my son full time. She's been mostly living here since quarantine, and we've both been working at home. About 5 weeks ago, I started working out of the house for 3 days a week and she has been staying at the house with my son since his childcare is still unavailable.
Her watching him is a huge help and I couldn't work without her unless I was going to put in a huge amount of effort into figuring out childcare. I appreciate her help so much. Our relationship is okay, she's much less independent than I am, and the one or so days a week that she's home are a nice break for me to unwind and just do my own thing. On those days she wants to be in constant communication- I think one day I counted 6 different "I love/miss you texts". It is nice to be wanted and loved, but also I find myself thinking "I've just been with you non stop for 6 days, yes, I love you but give me some time to miss you!" We pretty regularly come back to her feeling like I'm not as enthusiastic as she'd like, and I don't vocalize to her my own side because, well, when I do...
The issue: A few days ago she and my son are in my office hanging out with me, playing. I hear her tell him "Me and your dad are going to get married". For the record, I'm not a very reactive person, I need time to process things. But the fact that I didn't immediately express it doesn't mean that this didn't make me uncomfortable and give me pause. My perspective is very simple- That's something you say when you're engaged. It doesn't mean that I don't love her or don't see a future with her or don't want one.
Yesterday this came to a head at breakfast when my son said "Dad you're gonna make GF a [my last name]". I should have addressed it with her before, but the moment presented itself, so I told him that we had to do something called getting engaged before that could happen. Of course, she was unhappy and expressive. I should pause to note that she's an emotionally intelligent and articulate person and these issues don't turn into shouting matches or anything. Her reasons for justifying this are:

• We're going to get married eventually anyway, so what's the problem?
  
• He's 3, he doesn't know the difference between engaged and dating.
  
• She didn't explicitly say we were engaged, just that we were going to get married "someday" (That's not what I heard)
  
• Combinations, variations, and extrapolations on the above.
  

The thing is that I don't disagree with any of her justifications, except the difference in how it was worded. All true or else I wouldn't be in a relationship with her. But I still don't think that justifies telling your guy's 3-year old that you're getting married when you're not engaged or even close to it.
Currently She's at home and we're having a 36-hour cool-off time after this disagreement that left her feeling dejected and sad and unwanted. I will add that most of our disagreements are in this vein of me not being as enthusiastic about the future as she is. For example, she really wants to have a baby with me and while I'm okay with the idea of having another kid eventually, that's not where my mind goes to play, and that to her is rejection.
If it comes across that I'm a cold, unfeeling partner, I try really hard not to be. I want to give her what she wants and have a successful relationship, but I also believe it was inappropriate to tell my son that we're getting married.
We don't see eye to eye on this and I am really blundering communication when I try to express myself. Am I wrong? How can I express that I want to keep things stage-appropriate and that even though we're not there now, that isn't a sign that I don't love her and we might get there eventually?
tl;dr: GF Told my 3-year-old that we're getting married. I feel Yikes, that's a lot right now, and I don't know how to verbalize that that isn't appropriate without detonating a lot of feelings.


Source.