We first discovered Tarn Hows before we had children. We parked near Coniston and walked up to Tarn Hows, followed the circular walk around it and then walked back down. It stuck in my mind as somewhere to visit once our first daughter came along and we found ourselves back in The Lakes with a 6 month old. Now we park up at the top near Tarn Hows instead of down at Coniston though.
We love visiting Tarn Hows when we are in the Lakes. Not only is it a beautiful walk but it is very accessible for young families. There is a national trust car park (£4.50 for 2 hours, free for NT members) with toilet and baby changing facilities. The walk around Tarn Hows is just 3 km and the path is buggy friendly. Although the path is suitable for prams, be aware there a few short steep sections. We visited whilst we were in the Lake last October and it took us just over an hour to walk around but this is at the pace of a 3 year old who likes to stop to pick up leaves, sit on benches or climb on rocks that she thinks are mountains. You can only begin to imagine her reaction when we discovered cows on the path!
You can find full details of the walk here. The photos below are a mixture that I have taken on our visits over the last 6 years (hence the different weather).