The correct option is C they are able to keep children's attention for long periods.
Option C: As mentioned in the given line, "Considering the low attention span of today's kids, toys with higher play value are able to engage them longer.", the toys with higher play value are able to engage kids with low attention span for a longer time. So, those toys that are able to engage them longer have a high play value.
This is mentioned in option C, hence it is correct.
Option A: This line "Designing toys for children is challenging yet stimulating." is a general topic line and does not relate to the question.
Option B: This line isn't implied by the passage.
Option D: Similarly, this too isn't mentioned or implied.
Hence options A, B and D are incorrect.