But here is one reason why there is a problem with that. The fossil record all over the earth shows evidence of animals like dinosaurs having diseases like cancer, and of course they must have died to produce the fossils.
However when God made everything He said it was good, and there was no death or disease before Adam sinned. Those things came as part of the curse, after that first week of Creation.
So it doesn't add up to say that those millions of years happened before or during the 6 days of creation. The fossils had to have come afterwards because of the evidence of death and disease.
And as Ken Ham loves to say, don't you think if there was a worldwide flood, there would be 'billions of dead things laid down in rock layers, all over the earth?' And that's exactly what we have - billions of dead things laid down in rock layers all over the earth - the fossils.
If you want to know more about Creation vs. Evolution, Answers in Genesis have a great website for kids with videos, and you can find out a lot from the video series 'Dinosaurs, Genesis and the Gospel' as well as all their other videos of course!
Part of 'For the Searching' on Sundays