In science, it's not just about the difference in content, it's also about the style of question asked- the foundation paper tends to have shorter answer questions, easier language and more multiple choice style questions. Higher paper has more 6 mark questions and breaks content down a lot more.
I do think there's a case for some kind of intermediate paper or allowing mixing of tiers in combined science- however, this makes things even more complicated and is unlikely to happen when there's a current move away from tiering altogether. However, equally, there's no real benefit to getting a 6, unless you're going on to further study, and then you should be easily capable of high tier.
A child who rushes and makes lots of mistakes in science is probably better on Foundation, as the questions are broken down more and they'd be forced to stop and think a bit more about what they were writing. They'd also be better off learning some proper exam technique, rather than gambling on getting the right questions on higher.
I think most teachers do know what they're doing (now) and so it's best to trust them unless you're really sure a mistake is being made.