I've used PlantStep for years and can't imagine doing my daylily info without it. There are a couple of ways to track seedlings on there. In the section that Peggy and Becky referenced you can plan your crosses, track when you harvested the pod, when it went into the fridge, when you planted it and then put in pics of the seedlings. If your seedling is from a registered cultivar that you have, you can add seedling pics to the pics' section for that daylily so you kind of a genealogical library of what that plant is producing.
For example, William Marchant has been posting pics of the seedlings he's getting out of Viva Glam Girl. I have the plant but it's new. This is the first year I've worked with it. I like kind of knowing what it's kids look like even before I get seedlings from it so I'm using his pics for now.
If you were wondering about "naming" the seedlings I have a couple of suggestions. Unless the seedling has a garden name (like the morning I came out and found a bloom that was over 3 ft tall and a gorgeous poly...well, the garden name for that one is "Holy $&*%" because that's what I said when I saw it. Sorry, I digress. When seedlings bloom and I want to make note of them I give them a number. The first of it is the year's number and then I just start with 1. The first new seedling that I see this year and want to keep with be named LLM-151. The first three letters are my initials. I've done the numbering different ways but this is easy and has worked well.
When I give the seedling it's number I take a pic of the number with the bloom so I can easily ID it later. This one is from an older seedling but you get the idea.
Another way to ID your seedlings is to give the cross a number. This works well if you're doing long crosses. Say you cross Blue Wonder to Feeling Blue and get lots of seedlings from it. You could name the cross 151 and as you pic plants to keep from that cross you just go through the alphabet to differentiate them. The first one would be 151-A, the second 151-B, etc. If you run out of letters you're obviously choosing too many seedlings and we'll have to have another talk.