This is not taught often as it can lead to confusion and questions that do not concern us or our salvation. But a careful reading of the scriptures and some LDS church leaders would convey that Mormons do believe that God has made and populated other worlds and that Christ is the Savior to all of God's children and truly is The Only Begotten.
Doctrine and Covenants section 76:
22 And now, after the many testimonies which have been given of him, this is the testimony, last of all, which we give of him: That he lives!
23 For we saw him, even on the bright hand of God; and we heard the voice bearing record that he is the Only Begotten of the Father—
24 That by him, and through him, and of him, the worlds are and were created, and the inhabitants thereof are begotten sons and daughters unto God.
Further, in the Book of Moses, chapter 1:
31 And behold, the glory of the Lord was upon Moses, so that Moses
stood in the presence of God, and talked with him face to face. And
the Lord God said unto Moses: For mine own purpose have I made these
things. Here is wisdom and it remaineth in me.
32 And by the word of my power, have I created them, which is mine
Only Begotten Son, who is full of grace and truth.
33 And worlds without number have I created; and I also created them
for mine own purpose; and by the Son I created them, which is mine
34 And the first man of all men have I called Adam, which is many.
35 But only an account of this earth, and the inhabitants thereof,
give I unto you. For behold, there are many worlds that have passed
away by the word of my power. And there are many that now stand, and
innumerable are they unto man; but all things are numbered unto me,
for they are mine and I know them.
This is in harmony with the Biblical teachings in Hebrews, for instance:
1 God, who at sundry times and in divers manners spake in time past
unto the fathers by the prophets,
2 Hath in these last days spoken unto us by his Son, whom he hath
appointed heir of all things, by whom also he made the worlds;
3 Through faith we understand that the worlds were framed by the word
of God, so that things which are seen were not made of things which do