"Like toy soldiers" does mean disposable and easily knocked down, but in each context the use of the phrase takes on additional connotations.
For Martika (watch video), there seem to be multiple levels of meaning. First, there's the sense of young people in relationships being toy soldiers in that they often hurt each other (get knocked down), but keep going on. Some have argued the toy soldiers are the men the singer is going after as she breaks one heart after another. There are also clear drug allusions in Martika's video. In this layer, those who are addicted to drugs are the toy soldiers. Ultimately, I think the song is about the loss of innocence—whether to drugs or bad relationships, or both. Using a childhood toy as a metaphor conveys this well.
Eminem (watch video) didn't remake the song, but samples Martika's chorus as the chorus to his rap song. The following definition from the Urban Dictionary explains Eminem's use of the phrase:
Urban background, orginating on the streets and in hip-hop lyrics
A friend that's willing to go out and ride with you, no matter what the situation may be
A friend who has your back and is willing to give his life to ensure your protection (in essence, a friend that acts like a body guard)
My dawg G-War is my toy soldier...he always got my back
The song is about Eminem's attempts to deal with a violent feud in the rap community and the video depicts a member of Eminem's original group getting gunned down in a drive-by shooting.