It is my opinion that 1/64 scale has a couple advantages over the other scales. First the size would be easier for children to handle. Second is the enormous variety of 1/64 scale (non-railroad) models that are available. Just take a look at the toy department at Wal-Mart or just about any other store. This may be the ideal thing if your children have interests other than trains.

There are other things to consider as always. Its greater-than-HO size make it less desirable if space is lacking, and the standard 4x8 layout may be a bit tight for the trains. However an around-the-room shelf layout would remedy this problem. Also S scale offers considerably less variety of train models than HO and O scales. But this may change if it gains more popularity.

These are just few things to consider if beginning a model railroad with children in mind. And still just my opinion.