Being a beginner, I dont know if I would recommend jumping straight in with the 70-200mm F4L. Only reason I say this is that the short end (70mm) will be nowhere near wide enough to take snaps around the house etc, which Mausue will want if she is going to photos of her kids. If you have the budget to buy more than one lens, then it is a great chioce, but you will need something wider (unless you are getting the Kit lens - 18-55mm with the 400D).

You also said you wanted to do landscape shots. The wider the better for landscapes - The Canon 10-22mm is one of the most popular and is an awesome lens. You could get it for under £400 on ebay, but expect to pay more in a UK camera shop.

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