Just a few thoughts/ideas:
1) Check out ACME Planimeter over at http :// acme . com/planimeter/ which utilizes Google Maps and allows you to drop points to calculate area. Most accurate for larger areas as it uses spherical area math, but you may check it out to see how accurate it is especially if for example you know your lot is 3 acres or what not.
2) Really consider removing everything out of your old pond and starting new with known entities and quantities. You will never have to wonder or guess if you start fresh. Use the excitement of a new pond to kill those invasive cattails and weeds in your old pond and use that energy to research, research, research
3) It is much easier to place your structure in a dry pond or a very shallow pond. If you're able to clean out your original pond and in the Spring pump water from the newer, larger pond into the original pond, place your structure, and then pump back that may be a worthwhile consideration.
4) When you determine your game fish and table (eat) fish then you'll know your forage fish. From what you've posted thus far it sounds like bluegill and largemouth bass game/table, and fathead minnows may work well for your family (grandson included). The bluegills will bite, bite, bite which makes for fun kid fishing, and the bass will be a different animal for your grandson. If you start with a clean pond, then you can plan and manage your fish best.
You may consider stocking trout fingerlings in Autumn for the bass as they love to dine on them (lack the bluegill spines), and they are a great bait fish! You may consider channel catfish if you or your family like to eat catfish. They will compete with the largemouth bass for the bluegills as forage but you have not stated you're trying to grow massive toad largemouths. The catfish will also provide for night time fishing fun.
If you have or include some rocky areas you can add in crawfish for you to dine on and the bass to dine on.
Also if you're considering a dock to fish from, hang out and enjoy the pond on, etc., an empty or shallow pond is easiest to build that dock in. If you're considering a dock as a swim platform and a spring board then make certain to locate it in water deep enough for diving. Also locate fish habitats within casting distance of the dock for enjoyable dock fishing.
I inherited an established pond. With a tree-ridden dam, a warped and broken dock, a shoreline lined with water drinking trees and no spring to pump water into the pond, no fish cover / structure aside from vegetation near the shoreline and the stumps left in the pond bottom and the trees thrown into the pond, an overgrown trail road complete with water runoff trenches that made skilled navigation not an option, and no deep pockets to pay to rectify all of the issues.
You have the ideal, essentially a clean slate to address each and every line item before it's "too late". It is always more expensive after the fact.
