When SW boards families with small children, what do they mean by small children, up to 5 yrs old?

If I had "small children", do i have to check in at the 24 hr mark, or would i be ok to check in when i get to the airport and still get good seats?

We're a family of four with 5 & 7 y.o. kids. Id like for us to be seated together, or at least close to each other, at least 1 parent with 1 kid.

Thanks.