I think you just summed it up perfectly.
The best thing you can do is be an example and as the child grows up he will be influenced by you as an example. I have two perspectives on this though: 1) from an adult convert to Sikhi and 2) as a parent of a baby whose mother is not Sikhi.
First, I am Sikhi now, but I am 34 and have only been Sikhi for several months. I have always been "spiritual" starting as a child and have a long, maze-like, journey from being raised Christian to bouncing from atheism to Judaism to Buddhism (for the longest stretch) and now Sikhi. I wouldn't have been ready for Sikhi before now. I don't even think I knew about Sikhi until I was in my late 20's when I met a Sikh for the first time in my life and still didn't really know what Sikhi was. I still found Sikhi for myself though! This is Waheguru's plan for me - everything has a reason. The important thing now is that I am on the Sikhi path and for me to stay on this path.
Second, I have a baby and that means thinking about the future for my baby. My fiance (his mother) isn't very spiritual, but her family is Catholic and she wants to expose him to Catholicism. I'm OK with that as long as I am able to expose him to Sikhi, as well. It is completely non-traditional and I know many here will probably shake their heads about that, but I am OK with it. I wasn't raised Sikhi, but I am now and I think the path I took set me up to be Sikhi and as I said, I don't think I would have been ready at an earlier age. My main plan is to lead by example. If he chooses Sikhi at an early age - great, if he finds it later in life - good, if he never finds it this life, well that is Waheguru's plan and I will accept it.
As you said earlier, it's all Waheguru's hukam. I find myself saying that a couple times every day! I hope that my take from my couple perspective is helpful. I thank Waheguru daily for letting me find Sikhi - I hope that your new brother finds it, as I hope my son also finds it.
Waheguru Ji Ka Khalsa, Waheguru Ji Ki Fateh
-I.