Because it's at the very least a potentially historic event if we win and "I was at home in my PJs" isn't as exciting a place as sharing an environment with other people rooting for the National Team.
Mum, Dad and siblings jumping up and down in jubilation is nice but not the same as when a busy, happy environment with lots of people are celebrating. It's nice to get dressed up for the event with kits and faceprint etc, sing football chants with others, celebrate in a bigger way - for some families.
With lots of alcohol, loutish behaviour, people who don't like having kids in public spaces, there are a lot of venues which are showing the game on a big scale but not appropriate or safe for kids.
Several venues in my hometown are having "family friendly football screenings" where keeping things, "clean", "sober-ish" and totally safe for children is the priority. They're having games at half time, "characters" to visit the kids, mini discos with songs like Sweet Caroline and Freed from Desire etc for them to dance to...That sounds way more fun and yes, "child friendly" for many kids than being at home might be. One of the venues is the (church) school hall, so kids will be able to experience this with their classmates and friends in a way they wouldn't if all of them were at home.
Ultimately, you know what your kids would prefer and if it's staying at home, fair enough, but it's pretty joyless to declare that the living room is the most child-friendly place there is for every child. I mean, why do anything? Why go to the cinema when you have a TV at home? Why have a sleepover at someone else's house when you have a perfectly good bed at home? Why eat out when you can eat at home? Because sometimes it's more entertaining to do stuff outside of the house, and there are many reasons watching the England/Spain Final in a child friendly space would be more entertaining, fun, momentous, sociable than being on the sofa in pjs.
And I say that as someone who is neurodivergent and detests leaving the house.