I haven't written much since yesterday because I've been thinking. I feel like my story is lacking in adventure and depth. What I wanted was a bit of both romance and action. There is a scene I wrote in the beginning about a journey they would go through in order to free someone. Since I'm writing a series, each child would have to find and retrieve their own item. On top of their training and fighting off their fathers minions, I was wondering if this will all be too much for my readers? I was going to add this either way, further into my novel, but if this will happen then I can think of something else to do. Maybe rearrange things around then piece them back together. Or am I just worrying for no real reason?
Once you start writing, you'll find all sorts of things that need to be rearranged, rethought, or thrown away altogether. It's nothing to worry about. Painful, maybe, but it happens. You might be able to pull it off, but you won't know until you write the story. Worry about all the other stuff later.
If something like Game of Thrones, with its seemingly unrelated characters, settings MD plot points can be a critical and commercial success, then you have nothing to worry about in terms of confusion. I mean, as long as it's written well.
The only thing I'll add to @stevesh's excellent advice is this: if you insist on starting out planning to write "a series", don't defer something that might work very well in the first book because you plan on putting it in a later one. Write the best story you can up front and worry about the series later.