There's a hidden cost to having kids that many parents don't see coming

Having kids comes with a slew of hidden costs. However, one expense in particular surprises parents because so many people perceive it as 'free': Sending your kids to public school.

But it is free! Right?

Well, kind of. Just because you're not paying tuition doesn't mean you can simply check all school-related costs off your list. And school expenses can add up surprisingly fast.

"I think what makes it a hidden expense is because they're going to public school. Just that persona of public school - it's free, it's paid for," Pam Horack, CFP and "Your Financial Mom" at Pathfinder Planning LLC, told Business Insider. "So many costs there are paid for, definitely the basics are there, but there's a lot of extras that people might [not expect]."

Hidden costs can include everything from school supplies and booster club donations to technology fees and matching t-shirts for field trips. It's always something.

Horack gives an example: "Everyone in our school district gets an iPad. They don't have to pay for the iPad, which is great, but they do pay for the insurance on the iPad."

In the school district Horack's children attend outside of Charlotte, North Carolina, middle schoolers also go on overnight field trips, subsidized by their parents.

Of course, parents can always opt out of participating, but pressure from both the school and their own children makes it difficult to say no every single time.

Luckily, there are a few things parents can do to plan ahead and avoid being sidelined by these costs:

1. Call your school district

It never hurts to know ahead of time what type of fees you can expect for the upcoming school year, so don't be afraid to ask.

"Especially if your child is going for the first time or you're in a new school district, check with the district office and ask them, 'What kind of additional fees can I expect to pay?'" Horack recommends.

2. Budget for "school costs" - even you don't know what they are yet

Seasoned parents know that a new school year means there needs to be room in the budget for binders, pencils, and a PTA fee for two. But other things always come up, especially the first three months of the year.

"Especially as they get older, are there sports fees? Do you have to buy a new uniform for band? Is your singing group going on an overnight trip?" Horack says. "Think about those type of things that your child might encounter throughout the year and go ahead and incorporate that into your budgeting so that when the time comes to pay for those things, you'll have the funds available to pay for it. Even if you don't know about it ahead of time, have the funds available - it'll come up."