Jesus warned his disciples of having unbelief, we are to heed his warning.
The disciples were not able to do the task at hand; to cast out the devil from the child; because of the lack the faith, because of their unbelief. One small grain of mustard seed is very powerful. When it is planted, the plant will grow to it's fullness, it has many seeds at harvest time. It multiple.
So with a mustard faith, whats so ever you ask, in faith, beleive. Speak it out in faith, nothing shall be impossible unto you.