It's easier to find sentences above 7 year-old-level (e.g. Simple English Wikipedia), but the lower the level of a sentence, the harder it is to find in an massive scale. For example, 3 year-old-level sentence is "The cat sat.," whereas 7 year-old-level sentence is "Birds can be many different sizes. The largest bird of all is the ostrich." I'd like this dataset to be more or less uniform in the volume in terms of the difficulty (e.g. not too many 7 year-old-level ones compared with the 3-year-old ones). There is a list of vocabularies appropriate for each age, so I can filter a sentence by the used vocabularies and the length. 

I don't think the dataset satisfying this condition currently exists. I'd very much appreciate if you can give me an advice how to construct this dataset by collecting the samples from the Internet or by any other means.