Let me explain how to accomplish this, then once you know that, you can do it and save the preset. First, take a subject similar to one you might use. Here is a baby image that looks good for a pastel treatment:
Note that the contrast and tonality are pretty good and that the detail is good throughout. Now, in the Lightroom Develop Module, slide the fill light slider to the right. This will, all by itself provide a certain amount of pastel. I used a setting of 50-60 and what's really happening here is that the shadows are being moved into the lower third tone range and highlights are being allowed to blow out. (You may also want to lower the clarity to reduce the local contrast.)
Second, most people who are doing this are using some color overlay. Either a rose or yellow. You can do this using the Lightroom Graduated Filter. I used a very light red to provide a warm overlay effect.
If you like the look, then save it as a preset and you're ready to go. Moving off the "what you can do in Lightroom" topic, there is "what you can do using 3rd party filters". Here is the same image run through Nik Viveza to bring the white background more white and the Nik Color Efex Pro to provide the vintage look (it's about 4 clicks to get the entire effect):
All images are © Steve Ross (me), just so there's no question whether I ripped someone else's stuff off.