llGetLinkKey()
llGetLinkName()
llGetLinkNumber()

And that's all. On the other hand, you can totally alter a linked child from the root prim:

llSetLinkAlpha()
llSetLinkColor()
llSetLinkTexture()
llSetLinkPrimitiveParams()

What I would like to have in LSL is:

*llGetLinkAlpha()*
*llGetLinkColor()*
*llGetLinkPos()*
*llGetLinkPrimitiveParams()* (This one would be the bare minimum.)
*llGetLinkRot()*
*llGetLinkTexture()*
*llOffsetLinkTexture()*
*llRotateLinkTexture()*
*llSetLinkText()* (Would be very useful for texts on HUD buttons!)
*llSetLinkTextureAnim()*

And eventually --though we already have llSetLinkPrimitiveParams():

*llSetLinkPos()*
*llSetLinkRot()*

Why? Because the only way to move/rotate children prims is either to hardcode their positions and rotations in the root prim script(s), or else to add a script in every of the children prims. IMO, it's very illogical that the root prim can change the position and rotation of its children but that only them can determine their relative position and rotation. (The same applies to the alpha, color and texture.)