There is one objection against innocent babies being written into the book of life since conception. The fact that they have made no choice to be so. At least the Israelites had made the choice to enter the old covenant. Ultimately, everyone must accept the Lordship of Jesus and receive his nature before they can enter heaven. Noone is born with that nature, it has to be received. So even though babies are accepted into heaven they still have to accept and receive that. It can be argued that only then are they written into the book of life.
Can they reject ? The only answer to that that I can think of is "why would they"