You genuinely have to work at it. I am always baffled by people who think you shouldn't have to. We have to work at anything that we want to stay good over the decades - our health, our homes, our careers. The same is true of relationships - with family, friends, children and spouses.
If you still really appreciate your DH, which you seem to, I can almost promise you that you will fall back in love with him at some point, and it will take you by surprise when you do.
I'd start by doing small everyday things and planning some fun together. The every day things should foster closeness - maybe play music after work or in the mornings, that you used to love when you were first together. Or do things you used to love doing together - go to see a band or comedian you both love, live. If you used to go skating or hiking or bowling, do that. But also do new things together. Create some achievable small projects that will make you feel good - DH and I have done things like climb UK's highest peaks together (with DC); we went on a carpentry course and built some shelving for the house (without DC!) Discuss and plan for a really exciting family holiday and each agree two or three highlights you want to experience, then book them for each other. Take turns to organise a date for each other. Maybe join a local campaign and work to help achieve it. Or both sign up for a fitness challenge.
Have a chat about niggles and coldness, and agree to be warmer and kinder and sillier together so that indifference and disrespect don't set in. Say nice things about each other to the children, within each other's hearing - Daddy's got a lovely singing voice, hasn't he? Mum is great dancer etc. Notice how each other are looking and give compliments often. It might seem performative and false at first but after a while, it becomes second nature.
Over the years a lot of people have commented how polite DH and I are to each other. Always thanking each other for small things or checking if the other person is happy with something. But I see it more as considerate than polite.
Keep conversation alive so it doesn't descend into basic practical stuff about the children and house. Share weird stories from the newspapers, ask each other's opinion on political stories or local news issues. Make each other laugh.