They have no clothes at their dads, just pack a bag from my house that's it.
Take this up with their father, your apparently lovely ex who you have such a cosy relationship with you know the details of his finances.
I can't just stop buying her child present even though we're not related, it's odd and I can't say no to my children when they ask if they can buy her a present.
A big part of parenting well is saying no occasionally. “DC, if you want to buy x child a present please ask your dad, he can take you shopping”. Parent your children 
I just know if I was a stepmum I would be different.
Don’t be ridiculous. You have no idea what you’d be like as a step mum. You’re ignoring the multitude of step mums bothering to reply to your thread and explain that presents for their step kids are joint by virtue of their marriage of committed relationship joining their finances.
Like the bike DS wanted, she had nothing to do with it.
How many adults does it to take to choose and buy a kids bike? Why’s it a failing of hers that she had nothing to do with it?
But yeah I guess I just have to accept whatever presents my children get from that household
Well, yeah. He’s the parent to these children in their household. It is literally nothing to do with you how they arrange these things.
Seriously, you’ve been told, repeatedly, that’s it’s not your business, the way they do it is normal and you’re incredibly and weirdly over invested.
Focus on your own household, your own finances, your kids when they’re with you and your own life. Your ex and his wife owe you nothing and you won’t do yourself any favours obsessing about this.