YTA. I honestly would've said you both suck, but you were okay with him just flaking for "me time," so it's not actually about him missing her cupcakes. Plus, I honestly don't know if the 4 yo is going to care her stepdad is not there on another day of thousands when he's there most others. So he's in the clear.

You however, Mom, are disregarding:

1) Someone's responsibility to people they've already made plans with. This sounds like you did some last minute planning and decided that you have unmitigated access to his calendar, past present and future.

2) You call it "just bailing on his friends." I don't know who they are but he may have known some of them longer than you, not that that matters. Just because you matter a lot doesn't mean they don't matter or that they shouldn't matter compared to spur of the moment plans you put on him. But yeah, he's not just bailing on you. He's keeping a commitment.

I always look at planning conflict in terms of "who is going to be more disappointed." I don't know the details of hanging out, but it's more than 1 person. So to hang out with you and the kids, he'd have to actually "bail" on 2+ people, so that you feel like he's fulfilled his unspoken obligation.

So yeah, unless you promised to your daughter that your BF would be there on his behalf, all you have is that you're disappointed in him for him flaking, but not to hang out with people he's already promised his time to.