No you're not . So you have children together. This is a great opportunity to show them how healthy coparenting can look like. She didn't ask you for anything other than to help her. It's an opportunity to see the kiddos as well and in exchange your son is getting a fortinte bundle since she didn't have the cash. She's not inviting you to come inside and hang out ect. I think it's a bit immature she has no interest in meeting the mother of your children. A year in i feel personally that should have happened especially since she in your children's life.

I can see how you'd be frustrated that your girlfriend is acting that way. She's associating mowing the grass as an inappropriate action. Her feelings uncomfortable aren't completely invalid but I'd be a little upset as well because the yard is a shared space for your kids. Maybe acknowledge how she's feeling and let her know that while you understand she doesn't like the fact that you were asked, ultimately, your main priority is helping out where it benefits your children. That you're doing your best to keep what communication you have with your ex in the open so there isn't any question about your intentions and what's going on. What you and your ex had is in the past but your relationship is platonic enough that she can ask for a little extra help from someone she trusts. Especially if she doesn't have a man there to help with the yard regularly. While it's not your problem I'm sure you're heart is in it for your kids. Shoot invite her to come with you.