For some time friends of mine have recommended to me the Anki flash card system; Anki is an implementation of the Spaced Repetition Learning System: a system of learning based on common-sense principles such as showing at greater frequency cards with which the learner struggles, and less frequently those that have been mastered.
Using this system, it seems that my daughter gets a well-balanced mix of words she's already mastered, words she's learning, and brand new words, and in the end that seems to help keep her in the optimal state of flow while learning ! This is corroborated by her being excited to do the flash cards each evening.
After using this system 15 minutes a day, in just ONE WEEK my daughter has mastered all 100 of the sight words her teacher had assigned! I am so proud of her!
If you'd like to learn more about the Anki system:
It runs on all the OS's and devices, including iOS, Mac OS, Android, and the web. (the developer seems to fund his efforts through iOS sales, as that is the only platform for which he charges, and notably, a charge I was happy to pay.)
-  - Flow is the"... mental state of operation in which a person performing an activity is fully immersed in a feeling of energized focus, full involvement, and enjoyment in the process of the activity" [Wikipedia]. One of the requirements to reach "flow" is an optimal balance between challenge and ability.