i wouldn't put any good eggs that I really wanted to hatch under a silkie until she was setting tight, not budging and pecking my hand if I reached under her. I would think a silkie would not go truly broody until she at least laid an egg, but I am new to chickens this year, and as ready to learn as the next guy. Six weeks old seems VERY unlikely to set and stay there three weeks and hatch an egg. I have had week old chicks behave like you describe with day old chicks, hooking them under them with their beaks, looks like they are trying to protect them. Mine that I saw do that are bantam cochins, another variety known for strong mothering instincts.