What do you think Lois Lowry meant when she said this book is for all the children to whom we entrust the future?

The line you mention is the dedication of the book.  I
think that the author dedicated the book in this way to show that she means the book to
be a warning to people.  She hopes that children will read the book and be inspired to
prevent our society from becoming like that of Jonas's
community.

In this book, the author is warning about the
dangers of trying to make our communities too homogeneous.  She is trying to show that
making everyone be like everyone else (and taking the danger out of our lives) is
something that will make us less human.

By dedicating this
to "children" she is trying (I think) to emphasize that the book is meant to tell us
what is valuable in human life and society.