zu wenig - too little
zu wenige - too few

zu wenige - was? Es kommt darauf an, ob das folgende Subjekt zählbar oder unzählbar ist.

also

too little rice
too few children
?

ist richtig, wobei Kinder auch zu klein sein können...

Genau, und wenn es sooo wenig Reis ist, dass man die Körner zählen kann .... ;-D

We have some rice, but there is too little of it.
There is a child here, but he is too little.
There are some trees, but they are too few to be a forest.

Generally speaking, if you could in principle count them (regardless of how long it might take) they might be too few. If you are talking about one thing, it might be too little, although "too small" would be more usual. And if you are talking about a material or a substance, then there might be too little of it, i.e. if its quantity is too small.
Anything which is singular (e.g. rice) cannot be "too few".
If, in German, you could subsitute "zu klein" for the "zu wenig" without changing the meaning very much, then "too little" right and "too few" is wrong.
If you can understand the difference between the following two xentences, then you will have mastered it: "There are some children here but they are too little" vs. "There are some children here but they are too few".
In the first case, each individual child is too small; in the second, each child may be big enough, but the number of them is insufficient.
I hope this helps.