At some point many of these kids are going to find out and want to know what happened to their real parents. I strongly recommend that these kids know that they are adopted or fostered as soon as they are able to understand, even if they didn't ask. This will reduce possible resentment for not being told, if and when they do find out on their own.
But I WANT my kid to think that I'm their real parent!
Although this is noble, it's not always necessary. In fact, adoptive and foster parents can have an even deeper bond with their kids than their "real" parents. This because adopted and foster children were chosen, whereas biological children were many times accidents!
It is also common for adopted and foster children to have lower self-worth because their parents didn't "love" them enough to keep them. Even orphaned children may be (subconsciously) angry or resentful of their parents for not staying alive.
Make sure they know early on, even if they don't ask. The younger they are when they are told, the easier it is to absorb.
Stress that they were specifically chosen because they were special.
Give them a strong foundation of love, and you will fill the role of "real" parent completely.
Should I let them meet their real parents?
As long as the real parent(s) do not pose a safety threat, then yes. Support any interest in finding out what happened to their real parents. Meeting the real parents provides valuable closure and acceptance - regardless of how it goes.
Summary:
All things considered; blood parents are ideal but not essential. The quality of primary care giver is most important whomever they are. Somehow someway, most adopted and foster kids eventually find out, and want to know why they weren't good enough to keep. Make sure they know that they are especially loved because they were chosen, and not bestowed.