I try to encourage independant learning, which forms part of their gradings. The more impatient someone is to grade, the more they need to learn patience. So I would not spend additional time coaching them over another student. My younger cousin started training under me about 18 months ago. Because he was very keen initially, he was able to grade to Yellow Belt after about 6 months. However, since then he hasn't really progressed. A lot of kids seem to expect that if they just turn-up each week they will keep grading upwards! It may be harsh, but my time teaching is devoted to any student (regardless of grade or ability) that is keen to learn and wants to put the effort in. If a parent is unhappy with the childs progress, tell them to ask their child if they would still train if they never got another belt. If the answer is no, then they are not mature enough to train!