What's the best way to add multiple tracking pixels in multisite?

I have a multisite installation and need to have 2 different tracking pixels. One for one site and the other for 5 sites.

I'd set up the first one as a child theme and just put the pixel in the footer - works fine. I created a second child theme and put the other pixel there. Same exact method, but produces an error in the top menu (not changing the header at all).

So trying to come up with alternatives. Any chance there's a plugin for handling this? Would be nice for the client (and for me - really busy right now).

And any reason you can't have two child themes? Never tried that before, but seems like it should be possible.