I talk to them. I work a lot (single mother, very little involvement from their dad for years) - but I make sure that when we're together, I pay attention to them, engage in the conversation.
Yes, there are times when I'm stressed, or in pain, or just need quiet, or I know absolutely nothing about the subject but most of the time I can find some way to engage. And that's on anything from the imaginary history of the current computer game they're playing, to the best filling for a swiss roll, to whether we could paint the windows to look like minecraft glass, to whatever the hell their brains come up with next that they intensely want to talk about.
Actually this really helped with my rubbish small-talk skills, so it wasn't all to benefit them :) That gives us the shared connection even though in this modern world I'm working, they're both at school, and we each watch entirely different things on our devices so have completely different interests from each other.
Then as far as having their back goes - they know they can talk to me about anything and I'll be honest. They know that I won't immediately run to the teacher, but that I do reserve the right to do so if I think it warrants it (and I've done this twice, when it absolutely did, and my interference fixed the issues). When they started seeing their dad occasionally, I made sure they had a way to contact me, and I made sure I always answered when they did, and occasionally I went and got them when they asked. I made sure I was reliable.