They would've still learned Tom was a Controller, tried to free him (reluctantly in Marco's case) and tried to acquire battle morphs somehow (maybe Jake asked Cassie if he could pet some dangerous animals at the zoo, though without the guards chasing them I wonder how he would get the tiger morph, if at all). Once in the Yeerk pool, either the Animorphs are captured by Yeerk security when the two men spot them, or they do morph but don't have enough firepower and get captured or killed. Either way, Earth is screwed.
My what if: What if Visser Three had succeeded in making Controllers of the presidents and prime ministers of Russia, France, Britain, Japan, Germany, and the U.S.?