If you don't slope the bank, nature will do it for you. A gentle slope is easier to maintain (mow) and would be safer for the children. Call the Pond Boss office to order Lusk's book "Perfect Pond, Want One?" He discusses many aspects of pond design that most of us would not think to consider. If you even suspect that those boys would find their way to the pond unsupervised, delay construction until they're older. You know that nothing is worth that risk.
You'll want depth and a bloom, or both, to discourage the growth of vegetation from the bottom, although you'll want vegetation along the underwater slope of the bank and maybe a mound or two or three in the pond. Lusk's book will detail bottom design. Starting from scratch gives you a great advantage, so do it right. Having power will even allow you to aerate. You won't need to be concerned about needing water from adjacent acreage to keep your pond full. I don't know how you'll handle overflow, maybe just let it go where it naturally goes.
Get Lusk's book and do it right with no regrets.
