If you're using YNAB, you're presumably following Rule 2 embracing your true expenses. It sounds like you're budgeting down to zero without accounting for the irregular or less frequent spends. Have you got categories for birthdays, meals out, school events? Because they keep happening, so not budgeting for them won't help.
You should be aiming to let funds build up in these categories - by way of example, I have categories for things like TV licence and Christmas, which I put money into all year round. Plus things like car maintenance, which I aim to keep a certain amount floating in, so I can cover most car repairs without having to find the money from elsewhere.
That said, if you don't want to budget for eating out, that is entirely your choice, but you will need to plan ahead - bring the kids cartons of juice from home, or water bottles, or offer to host more stuff at your house (not sure that will work out cheaper, though!).
If you've been YNABbing for a while, I would have a look at how much you're spending in certain categories. I assume when something crops up like a school event, you're recording the expense, how are you categorising it? Ideally you'd be proactively budgeting each month, not just copying the values from last month?