There are three major requirements to avoid borer. The three are (1) taking the desired element (ochel) from the undesired (pesolet), (2) not using a utensil that enhances the sorting process, (3) using the ochel in the short term.[1] Although using your fingers satisfies requirement (2), you still fail requirement (1) because you remove the undesired milk. You may choose any of the following ways to rectify the situation:
1. Remove the cereal from the bowl, leaving the milk. This simply eliminates the problem that we mentioned by taking the ochel from the pesolet. You could even use a spoon to do so, as the spoon is not specifically designed for sorting/straining, and, in theory, your fingers could be just as efficient as a spoon.[2] You must be careful, though, if you happen to take a spoonful that has too much milk in it, that you don’t rectify the situation by purposely tilting the spoon to make the excess milk fall out. You are allowed, however, to take the spoon out of the bowl at an angle from the start. Depending upon how much your baby eats, this system might be tedious.
2. Have someone drink the milk you pour off. The problem you have stems from the fact that you are treating the milk as pesolet, something not to be used, at least in the short term. However, if the baby or someone else drinks the milk in the short term, then the milk is considered another type of ochel, and it is permitted to remove one ochel from another to eat immediately. Even if someone removes an element that he does not desire and does not eat, if he takes it out to give to someone nearby who does want it, no violation has been done.[3]
3. Remove some cereal along with the milk. The Taz[4] says that if one wants to remove a fly from soup, where the fly is obviously the pesolet, he may do so by taking out some soup along with the fly. The reasoning is as follows. The prohibition of borer exists when a person takes one element from another, not when he takes a certain percentage of the two elements that make up a mixture from the rest of the mixture in a manner that changes the relative concentration. The chidush[5] of the Taz is that even if by removing a particular percentage of the two elements only one element (fly-less soup) is left, it is still permitted. That is because halacha is concerned with that which one removes, in relation to the original mixture, not with that which remains.[6] Since there was a mixture of soup and a fly and one is removing a mixture of soup and a fly, there is no problem. Although some disagree with the Taz, the Mishna Berura[7] and a broad consensus of poskim accept his view. Your situation is even better, as you will be leaving some milk in the bowl, just at a lower concentration. You must be careful to consistently remove the milk and cereal together. If you spoon out some milk by itself for a few seconds, then the fact that you subsequently throw in some cereal will not retroactively fix the previous act of borer. If you use this system correctly, it is not considered a process of borer at all, and it would therefore not make a difference whether the baby eats the food shortly thereafter or significantly later.[8]