You decided to both give up work and study. Good for you, and I hope it will bring the rewards you both wish for.
When I was growing up, (Primary School) my dad was setting up a business, working 2 jobs to keep things going. OK so we weren't the poorest, but I do remember others having more than us. For a while.
By the time it was time to go to Secondary, things were better, and did get better and better and we lived well enough. There were still people much better off than us. We STILL couldn't go on EVERY school trip. There WERE no extra curricular lessons/activities.
What you and your DH are doing right now is building for the future, you made the decision, rightfully or wrongfully to put your family 'below the poverty line' for future advancement.
You want your kids to have the things you'd like them to have. NOW.
They don't NEED them NOW. There is always time to learn... as you and your DH are demonstrating.
WHEN your decision has started to pay off, you will be able to afford to pay for EXTRA-curricular stuff.
This time last year, i could barely make the rent. There was no way I could sign DS up for anything. Now I can afford to pay for after school stuff, swimming etc.
for whatever reason you and DH didn't study at uni when others did, but now you are going back and getting a higher education. You need to understand, as do your DC that you simply can't do it all, and do it all now. The time will come. Be patient. You do at least have HOPE of it getting better, many here, as you have seen, don't.