With 6 days, I would only stay in Reykjavik the first day. That will allow you to see the city and get over any jetlag, and then you will be ready to go the next morning.
The Golden Circle is an easy day trip, but I would not head back to Reykjavik. Instead, stay somewhere in that area or just north of there, around Hella. Then the next day continue on up the south coast and stop again for an overnight up around Vik if you have time, or if you are running short, then around Hella. The next day head up towards Jokulsarlon to see the glacier lagoon and diamond beach. Then make your way back and head up to Snaefellsness.
I love Snaefellsness and there is a lot to see on the drive there. I would stay the night up there, either Grundarfjordur or Stikkisholmur. The south coast and Snaefellsness are totally different experiences. Snaefellsness is sort of like a compact Iceland and you can see so much variety in this area.
The Blue Lagoon is wonderful, IME. We have been there on 2 of our 5 trips so far. I do not consider it expensive, but that is all personal. If you go when they open in the morning, you will easily avoid the crowds. I would do BL either upon arrival before you drive to Reykjavik, or on your last day just before the airport, depending on your flight time of course.
Don't forget the local pools. In Reykjavik, we like Sundhollin. It is about $8 US dollars pp, and less for kids. The local pools are very family friendly. In the Golden Circle, there is the Secret Lagoon as well as Fontana which are both quite nice and very family friendly as well. Secret has pool noodles, so that might help with your toddler a bit, but Fontana has a handful of pools including a super shallow one basically for little ones.
We head back for our 6th trip in a month. There is only one thing in Iceland that I felt was overrated, and you don't have that on your list and wouldn't do it with a toddler anyway, so no worries. Just go and have a wonderful time!