Frankfurt Airport to Heathrow

I don't think I understand the question. Do you mean how hard is it to get from the check in counter in Frankfurt to the gate, and from the gate to the exits in Heathrow? Not hard at all. Both airports have excellent signage. They also have online maps if you want an advance look.

we would be flying from USA to Frankfurt and then taking a plane to Heathrow

I'm confused too. Why fly to Frankfurt if your destination is London? Next...why would you think it would be "hard" for 3 women to "navigate" Frankfurt airport? Would you ask that question of 3 men?

If you are flying 2 separate tickets to Heathrow from the US (as opposed to buying one ticket whose itinerary takes you from the US to Heathrow via Frankfurt), allow 4 hours in Frankfurt in case your transatlantic flight is late, you aren't flying with carry-ons only, and/or the security line is long.

Have you already booked the tickets? If not, you should try to get the whole trip on one ticket. If you do this, you will have a protected connection; if your incoming plane is late, that is no longer your problem, but your airline's.

Taking two separate flights on two separate tickets on the same day is generally not a good idea.

You will have to go through immigration, pick up your bags, then check in for the second flight (possibly in a different terminal), and then go back through security and immigration to catch your second flight.
What is the minimum check-in time for your second flight? If your first flight is late landing, or you get stuck in immigration, or either flight is re-timed and you miss the check-in for your second flight, hard luck, you have no come-back and would have to buy a new ticket.

It would be a lot better to fly direct to Heathrow (or any other London airport), or at the very least book this as one through booking, then it is the airline's problem to get you on the second flight.

we would be flying from USA to Frankfurt and then taking a plane to
Heathrow

Again, I'm going to ask you to clarify this. Do you mean that you are flying to Frankfurt, visiting, then flying later to London? Or do you mean you are flying from the US to Frankfurt and then on to London the same day? Because I'm having difficulty understanding why anyone would do the latter.

This is all in a preliminary plan we are working on but you have clarified this part, thanks.

This is all in a preliminary plan we are working on but you have clarified this part, thanks.

If your itinerary is from the US via Frankfurt to London, it is hard to imagine there aren't other routes to London without changing planes in Frankfurt. If you want to visit both cities, you can use a multi-destination search function to land in one and fly home from the other. To find a quick, no-frills flight one-way between the two cities, try www.skyscanner.com which links to the many remarkably cheap European flights. Buy as soon as possible for the best price and be sure to read all the regulations on the airline website as they may contain extra charges.