Smiles i just want to say coming from a single mumma here what you are offering is truely amazing. You are a wonderful lady and friend.
Coming from a single mums point of view, if someone offered me what you are offering this friend of yours i honestly would never be able to repay them.
The stresses from day to day living without a partner is hell itself- not to mention tiring.

Just for someone to offer to look after my kiddies whilst i went shopping would be a dream. I really think your friend will appreciate this.
I also think you should start just by looking after them for the day- evenings would be a little overwhelming i would assume- for both her, you and your DH.