Mammals probably did not evolve from reptiles.  Instead they both evolved from an earlier group.  Birds did evolve from a specific group of dinosaurs and in many ways are still dinosaurs.

The reptiles are a convenient grouping, but they really are several very different and ancient groups -- turtles, crocodilians,  dinosaurs, snakes and lizards (the last two in the same group).

The problem I think some creationists who genuinely have problems with evolution (rather than just stubbornly insisting on their childhood teaching) is that they don't comprehend the time periods involved -- millions and hundreds of millions of years -- in such time periods a lot of things happen.  It is called "deep time."