Her parents knew about Beauxbatons - they'd seriously considered trying to send her there instead of Hogwarts, because her father was French - but they didn't know about the others. If they hadn't, Beauxbatons would have been a perfect choice, because unlike "Hogwarts School of Witchcraft and Wizardry" there was nothing in the title that said outright that it was a school of magic. It was implied if you spoke French, yes, but one could argue easily that batons did not mean magic wands - it could mean a conductor's baton, or something. She could have just told them she'd found a nice French school and pretended it was a Muggle school, they probably wouldn't investigate if she pretended she'd given up ...