I don't see the problem. Tell them their question is a good one, but one that's very difficult to answer while providing a simplified answer (if you have a toy in one hand, and nothing in the other and you put them together, how many toys do you have together?). Critical thinking skills can be taught in way that's age appropriate. What's important, or so I believe, is that young children are taught to be curious and that it's OK to ask questions and be skeptical. They won't always get an answer, but that doesn't mean asking was wrong.