Depends wholly what's in your contract. Sadly what your normal arrangement is, doesn't count unless it's written down.

If your contract with them states no fees due during school closure dates, then you can't charge them anything. If your contract states usual BAS rates apply on school closure dates, then that's what you can charge. If you contract states that care is provided for these two children on school closure dates and fees are due regardless of attendance, then you could charge them the full day rate.

What does your contract with this particular client state?

Without having your arrangement clearly in writing, you are limited in your options and all you can do is say "Thanks for letting me know" and accept you won't be working full days for this particular client and therefore you won't be getting paid.