Maybe think about why you want them to be outside. What are you wanting them to get from it?
There's nothing specially moral or virtuous about 'being outside' and yet a lot of people have internalised this message that it is better somehow - so maybe thinking about whether you have too, and whether you could just let them stay inside if they are getting the equivalent of what you want for them from outside.
fresh air? Open the windows.
exercise? Yes, they need some - so either being active - playing, dancing, whatever other activities indoors, or going to the park or swimming or a walk or whatever at some point, yes.
bit of sun on the skin? walk to get an ice cream or something
doing something other than screen time? Valuable. But reading, crafts, lego etc all fulfil that purpose, but inside.
learning about nature? can be done in other ways too.
looking into the distance for eye health? some walks or visiting an outdoor attraction or whatever, yes good idea
Not everyone enjoys spending lots of time outside - it can be too hot, sweaty, buggy, sticky, smelly, noisy, prickly or whatever, and so there might not be a reason to make them go out if they actually prefer to be indoors for whatever reason, as long as they are getting the equivalent of what you want them to get indoors.
But why make them be outside if you are going to let them do activities like reading or crafts or lego, if they don't actually want to be outside? It's no more virtuous to read in the garden that to read in your bedroom (speaking as a child who spent most of the time indoors!!). I did need to get more exercise, yes. But I didn't really like just having to be outside for the sake of it, and I still hate being too hot and sweaty and having prickly grass or tickly bugs crawling around when I'm trying to read!