Need help with Itinerary. First time in London, 2 moms with their 15 year old daughters for 2 days.

Your time and energy with jetlag will be somewhat limited. I suggest take the hop on and off bus to see Londons highlights.
Get off nr borough market and also any area u spot along the way that looks interesting. For a central wonderful but reasonable meal visit The Wallace Collection museum with cafe, it is so lovely in the restaurant atrium. The Wallace is a museum in a residential area so London, for sure. I visit every trip and entry is free.

If you want to go to tea then one of the best place in the city that won't break the bank is The Wolseley near St. James Park...just two doors down from Green Park tube station next to the Ritz Hotel. Cream tea is only 12.95 GBP and comes with tea and scones...it's a beautiful place to have tea and the girls might love it!
If you go to Borough Market take the time to go to Southwark Cathedral right next to the market...it's very small and very beautiful. You can eat at Borough Market...there are plenty of food stalls and small restaurants that are really good! Or you can just wander and eat samples...the cheese!! OMG.
Right across the river from Borough Market is the building with the Sky Garden...the only challenge is that if it's busy, you have to go put your name in at the beginning of the day to get an assigned time to return.
You really don't need a tour for the Tower...if you do take one then you should do so with a Beefeater who lives there and knows the history like no one else.
There is a mall directly east of St. Paul's Cathedral that has a glass elevator with a view directly onto the church...at the top is a beautiful roof deck with views of St. Paul's and that part of the city. It's free and amazing!

If your flight arrives at 8:30' there is no way you can meet a walking tour ay 10 am. You will have to go through immigration upon arrival, which combined with exit time from the plane can take a good hour or more. And then you need to make your way into London.

You really have one full day in London, plus whatever is left of your arrival day after transport from yeh airport. Prioritize what you want to do, and plan on a lot of walking.

Have some bad news for you... Borough Market is closed Sunday, and only open to trade on Mon/Tues, so unfortunately you’ll miss it on this trip. There is a smaller street food market on Sunday behind the National Theatre on the South Bank, that’s still worth a visit though and not too far from your hotel. Do plan a South Bank walk from Westminster to Tower Bridge, so much to see and do.

For a real treat/experience... make your way out to Spittalfields Market in Shoreditch and then on to Brick Lane a few mins further on. Endless street food, trinkets and fashion. Use Liverpool Street underground.

Monday and Sunday are not especially productive at Borough Market - but if daughter really really wants to see it there is a partial opening on Monday. It is completely closed on Sunday. You won't get the atmosphere there on a Monday that people go for.

90 minutes after landing you will still be inside Heathrow. No way you can be an hour away from the airport joining a walk. Sorry.

Tower of London is easy on your own, using the fabulous free (yes really, it is all in the price of the ticket) Beefeater tour. They really know their stuff and are very entertaining.

I'd skip the hop on hop off bus. Costly and often spends time stuck in traffic.

You could follow a walking tour from Rick's book. I agree that you don't need a guide for the Tower of London but make sure to do the Beefeater tour, it's excellent.

Jet lag can affect your plans. I wouldn’t go on a hop on hop off bus tour it will make you sleepy. You have very little time in London. Seeing the Tower of London can take half a day. I would pick a few sites and know you will miss somethings. That first day after flying can be a blur. I always think we will do a lot and then never do. Enjoy your trip

Maltby Street Market, in the Southwark area not far from Borough Market IS open on Sundays, and is really nice.

http://www.maltby.st

You may not be effected by jet lag, some people are and some aren’t. I’m never feel jet lag, we check into our hotel and keep busy all day. Your daughters being teenagers might enjoy walking down Kings Road with its stylish shops and punk history. The food halls at Harrods is fun to see. With only having a little over a day and half, you don’t want to spend hours at one location. You could go to Trafalgar Square, St.Martins-in-the-Field Church is across the street, then walk down The Mall or walk through St James Park you will get to Buckingham Palace. As you are facing the place if you walk around the corner to your left is the Royal Mews where you can see the queens horses and carriages. The tower is great but it would take several hours to see. Whatever you decide to do I hope you have a great time in the greatest city in the world.