What does the “the nails packed around it” mean?
It means exactly what it says. A terrorist took a bomb, packed nails around the bomb, and then detonated it. The word 'packed' suggests that the nails were tightly inserted between the bomb and some kind of casing. When the bomb exploded, the flying nails would have caused dreadful injuries.
What does " remember the bomb exploding" mean?
Again, this means exactly what it says. The writer recalls the moment when the bomb exploded. They also remember the nails that were packed around it and the carnage that this caused.
'Pastel pink' is a colour. It's a pale, whitish pink, associated with sugared almonds and baby girls.
The force of this paragraph comes from the contrast between the description of the present scene - rose garden/children’s storybook/pastel pink/sugar-white - which is gentle, sweet and innocent, and the writer's memory of the horrific events which had happened there.