The Question You're Actually Asking
You're not worried about flashcards. You're worried about the moment your child sits down in kindergarten and either has a foundation to build on or doesn't. "Math for toddlers" sounds like it could be marketing, and you're right to be skeptical of anything that smells like busywork dressed up as curriculum. So here's the plain version: number sense either gets built now, through hands and objects, or it gets bolted on later, through repetition and pressure, after your child has already decided math is confusing.
You can test this yourself. Ask your child which pile has more, five blocks or three blocks, without letting them recount. If they can't answer without touching every block again, they've memorized a sequence of words, not a concept. That's common, and it's fixable, but it's worth knowing which one you're dealing with before kindergarten does the finding out for you.
What you get from real math activities for preschoolers isn't a head start on arithmetic. It's a child who trusts numbers because they've already used them to solve real problems, dozens of times, with their own hands.