I'm a Slovak citizen; we're planning a road trip (by a private vehicle) in the Balkans focused on Bosnia and Montenegro in the end of July. Today I found out one of the kids' ID card expires in mid-October, which will be less than in three months' time when we reach the border of Bosnia or Montenegro. Unfortunately, that kid also doesn't have a valid passport and due to various personal reasons we can't apply for a new ID card or a passport before the trip.

I know the official rules of these two countries mandate the 3-months minimum travel document validity (even though it's not 100% clear whether it also applies to EEA ID cards, or just passports). But I'd like to know how it is in practice, since these two countries are quite welcoming, permissive, reliant on tourism and the border checks tend to be quick and brief.

Is there any hope that our trip can proceed as planned? Is it worth the risk? Or do we have to change our itinerary so that we stay only within the EU, where the minimum ID card validity is not an issue for us?

even though it's not 100% clear whether it also applies to EEA ID cards, or just passports.

It seems pretty clear for Bosnia, at least:

ENTRY REQUIREMENTS

Travel documents

An alien may enter BiH if he/she possess a valid passport or other identification document that may be used for entering BiH, provided that:

• it is valid for at least three months longer than the intended date of departure from the territory of Bosnia and Herzegovina, and
• it was issued within past ten years or is a document of permanent validity.

Source: https://msb.gov.ba/inf_za_strance/default.aspx?id=14555&langTag=en-US

I've crossed the Bosnian land border dozens of times. They sometimes don't check -- but some of those times have been in a rental car registered in Bosnia, so I certainly can't say what the probability of your family in your car being checked might be.

I have also crossed the Montenegrin border a few times, and had similar experiences, but, again, you won't want to count on being waved through the border. There's a very real chance that it won't happen. Even if you're checked, and they notice the expiration date, you might get lucky, but I think the only way to be certain of being admitted would be to obtain a new ID card or a passport.