Here is what I would if I listened to my brain: I would take the insurance job for the time being. Try it out. It is what you need right now for your child. You will be able to better support him financially and be there if he gets sick etc. Then I would keep my eye on further down the road when he was able to be a little more independent (teenage years etc.,) I would look into L&D. You will have been working as an RN for awhile it will probably be easier to find an L&D RN position. While working for the insurance co., take some online classes on L&D or do what you can to gain extra certifications.
Now, if I were to listen to my heart (instead of my brain): I would say go for the L&D job. You will always regret not taking the gamble. If you don't like it, then so be it, you tried it. You satisfied that long lived curiousity. If you can hold out and struggle for as long as it takes to get the RN position, it may be worth it. But like I said, either way, you will know and you can either move on or stay put.
When I was in a similar position, I went with my heart. The department I long desired ended up not being "all that". I was welcomed back to my previous job with open arms but still remain prn. I don't regret it. It was a learning experience and I never regret learning ANYTHING!
Good luck to you....I know you are in a tough spot and I feel for you. Let us know what you decide.