There are two solutions to this, an easy one and a hard one.
The easy one is to feed your child masses of protein and invest in a stretching rack. Buy some shoes with built in height enhancers, and persuade him or her of the merits of very long trousers dragging on the floor, or being the first one to wear shorts, or taking up synchronised swimming instead of cricket.
The hard one is to stand the child on a chair in its cricket spikes and trousers. Make some kind of mark to indicate the required length of trouser. Remove child from chair, remove spikes and trousers from child. Mend spike holes in chair. Have a large gin and tonic. Trim tape to length of perimeter of trouser leg. Get ironing board and put trouser leg onto it, fold it to required length with trousers inside out. The next bit is such fun that I am tempted to skip over it to allow you to discover the delights of the stuff yourself, but here is a hint - it is meant to adhere with heat and bond the surfaces together. In my experience it does this selectively - when you have the trouser leg in the correct position, it will stubbornly refuse to adhere and you will, instead, singe the material. On the other hand, when you accidentally mis-position it, it will stick like superglue and only pulling a hole in the trouser will put the two asunder.
The child should be warned, with a stern look and, if, necessary, a finger held threateningly in the air, that comments about anything being 'the wrong length', 'not straight' or 'looking stupid' will not be welcomed.
If you do go with the second solution, I suggest not feeding the child for several months in order to make it worthwhile having gone through the procedure.