There is an SKAction that sets the sprite's texture to another one - so you don't even have to remove your bomb node in your sequence, just replace the texture. You can use that in a completion block, which will be executed once your action is finished.
[bombb runAction:sequence completion:^{
[bombb runAction:[SKAction setTexture:explosionn.texture]];
}];
Or, if you want the entire sprite to be put in there:
explosionn.position = bomb.position;
[bombb runAction:sequence completion:^{
[self addChild:explosionn];
}];