Well it depends doesn't it?
If the party is in a soft play centre, or similar with a pizza at the end etc, each invited child will have cost the parents a tenner, sometimes even twenty quid per child. So I think a little box of chocs is a poor present.
If its just a little party in their home, these can be expensive to run as well, but it's usually less expensive per head, so in that case I'd say, if you want to brazen it out and give a box of malteezers from the pound shop, go ahead, BUT you might be judged harshly!
Of course if you know the parents well, you can say you're having a cash flow crisis, and will get a proper present for their little kid when you're rolling in the lolly.
For me, I go in with other parents and share parties (one party for four kids), and we ask for vouchers, one tenner voucher per invitee. So each kid gets about five book vouchers. Works out financially as well for everyone: parents share the cost of the party, and each child brings one present which are shared with the birthday kids.
Maybe don't go if you can't afford a present and play in the snow instead with your two boys?
It s not the end of the world not to go to a party if you can't afford a present.
Things change, it's not like well all be skint forever? Maybe have some old style family time instead.