True lip sync is quite difficult to achieve. The voice has to be recorded first, before the pictures. Then a frame-by-frame 'dope sheet' of mouth positions is prepared, using precise timings from the recording. The animation is then created from this list.
Fortunately there is a much easier way of giving the impression that a character is talking.
Make sure you have a few frames of a character's mouth moving in different positions. Children love creating endless mouth movements (and this is logical too, as they are trying to get enough animation to go with the length of the speech) However, you actually need only a few frames!
In post-production, loop up these few frames of chattering mouth. Every time the character speaks, use the chattering loop. When there is a pause, use a freeze frame of a closed mouth position. (Later, you can get more sophisticated, with the widest mouths being used for loud words, etc.)
It is essential that nothing else is animating in the shot at the same time! Otherwise, every time the 'chatter loop' happens this other unwanted animation will start and stop too. This is particularly important if you have a two-shot of characters talking to each other.
Partly because of this, close-ups are easiest and often most convincing.
If you don't have mouths for some characters, just a small amount of body jiggling as they talk, followed by a freeze frame, can work extremely well (and provides pretty cheeky animation too)