It's OK to plan, just as with all plan involving kids, it needs to be made in pencil because it cana nd will change.
Here are some of my thoughts:
1) The fact that she enjoys it is paramount. I'm glad to see that she loves running. It's a great sport that has much to offer young runners in every aspect of their life. Make sure that running always remains fun. That doesn't mean it can't be hard sometimes, but only if they embrace the challenege.
2) Conistency - that is a major key, be as consistent and sustainable as you can throughout the year. Young bodies adapt best to that which they do consistently. Their bodies are growing and changing and it will handle and adapt to that change best if you are consistent. Avoid big swings in volume or intensity, this is where most of the problems arise.
3) Capacity - one the beauties of coaching young kids is they have so much time in front of them. Start them on a path with small periodic increases, allowing plenty of time for their bodies to adapt and benefit before increasing again. Over time those increases build up and their capacity for work becomes bigger and bigger. This will help them in so many ways in the sport, but because the changes were also small and manageable they don't seem like anything to them.
4) Frequency - part of the gradual increases will be in terms of frequency. If frequency of running increases with capacity, it becomes sustainable for the long term. Frequency and capacity go hand in hand.
5) Mixture - especially at an early age, make sure she gets in a good mixture of work so that she develops evenly. Kids don't need to specialize - it' not either intensity or quantity. It's an appropriate amount of each.
That means some sprint work, some longer intervals, some tempo work and some long runs. Each with appropriate recovery days in between. (e-mail me if you want more on this)
6) Fundamentals - one of the greatest things we can do for kids to help them establish sound fundamentals and good training habits. More important than the times they run when they do a new workout type is the fact that they understand how to properly execute it and the fact that they have a set warm-up and cool-down routne that becomes a habit for them. Teach them a good drill routine to strengthen and enhance proper form. Kids love a good drill/strength circuit, its fun to do. Supervise this closely so they establish the proper technique and strengthen it. These fundamentals and solid training habits will last them a lifetime and greatly help them in every stage of their running career.
Rather than a set time frame, I suggest establishing a sequence that doesn't have a time frame but rather a list of next steps and then move forward to each next step only when she is ready and wants to move forward. There will in instances where she is going through a growth spurt and making a change isn't the right move, so you stay at your current work load for a few extra months. It's also important that she wants the increase. Her dedication and continued interest and motivation will determine when next steps are warranted. An unwanted increase is a sure way to overwhelm and sour a kid on the sport.
****** Be sure to be a good listener ******* You will learn as much from her as she does from you.
The coach and/or mentor roll is huge blessing to both you and the child. Good on you for wanting to take up this roll.
For a motivated athlete, the progression you suggest seems conservative but reasonable. If you should, or how much you should, modifty that will depend on her continued desire and motivation.