Most cake smash sessions we shoot fall apart, briefly, right around the moment the cake shows up. That is normal. Here is what actually helps.
Feed them first. A hungry one-year-old will eat the cake calmly and stop performing for the camera in about ninety seconds. A fed one-year-old treats it as a toy, which is what makes the good photos.
Skip the rehearsal. Do not let your child see or touch the cake before the shoot starts. The reaction on first contact is what we are there for.
Keep the outfit simple. One layer, nothing with buttons at the back. You will be changing them out of icing within twenty minutes.
Bring a wet towel and a change of clothes. Obvious, but the number one thing parents forget.