Being posed with the question as to "Why did God allow Satan to enter into the world when He knew Satan would tempt Adam and Eve?”, I understand that there may be more asking the same question. It is a good and valid question. First of all, we would have to ask ourselves why we would ask this question. Perhaps it is because we believe and know that God is both a sovereign and good God. For many, the understanding that God had more or less knowingly allowed Satan to tempt Adam and Eve under God's watchful eyes is hard to reconcile with God’s sovereignty and goodness. Why is this a concern for us? Now, I am assuming my readers are Christians, and that my readers' interest is not about the origin of evil or Satan. Given this, I will continue with our question posed.
We, humans, might think that God would use His sovereignty hundred percent of the time to protect is from our capacity to destroy ourselves (our intelligence, free-will, and self-consciousness make us very capable of ruining ourselves by choosing evil). Did God trick humanity? Has God entirely used His sovereignty to work out His goodness? It is here that our concern lies and it is a reasonable concern because it brings us closer to God's plan for us and gives Him glory.
We must take into account that the creation of the cosmos (which is not an eternal thing because it was created out of nothing [not made out of God, the Eternal], and stands apart [but not independent] from God) and humanity (comprised of free-willed beings), is a product of God's goodness and sovereignty. This glorifies God.
Think about life without choice. It would not be much of a life, would it? The meaning that comes from having a choice in life is that we can choose to do the right thing or the wrong thing; there is an alternative. Love offers choice. We must understand that free-will gives God glory as it allows Him to show His love (goodness) and His sovereignty at the same time so that we may choose Him willingly and not forcefully. If we pick against Him, God will interact in and with creation for His ultimate purpose (this is His sovereignty) so that He will give us opportunities to know Him for our benefit voluntarily. By using His sovereignty in this way, God shows His goodness and saves us from our capacity for destruction, in the sense that we may understand our need for Him. Often suffering comes from the freedom to choose. Without suffering, because of our fallen world, we would keep on sinning, only furthering distancing ourselves away from God (the Goodness); the Goodness as the very One who loves us. Suffering gives God another chance at bringing about His goodness to us through His sovereignty. Love would not do less.
Conclusion: God is so sovereign that He created the cosmos and allowed chaos to enter into it, and despite it, He works in a way that avails to us our choice (a display of His love). In this way, God can bring the chaos back into His order (a manifestation of sovereignty) so that His people will experience their need for Him and His goodness more deeply. God allowed Satan in the Garden because it was for God’s glory. That does not make God mean or selfish if anything it alludes to His goodness and sovereignty and the love, He has given to us: Our free-will and Christ Jesus to reconcile us to God (who is both goodness and protection). Love the beginning, Love in the end.
A good thing to note: Angels have free-will too and are created beings. The workings of those who choose to disobey, like Satan, God even uses to work for the good of His people, to display His love and mightiness.