I can see that this is old, but I'm gonna have to bring it back. I've got two sims who have been maxed on their friendship bar forever and the "become BFFs" option will not show up for them with each other. If they try to be BFFs with the child in the house, the option is available.
It might be worth mentioning their romance bar is nearly full as well, and I'm wondering if this is the issue.
I've looked everywhere for a solution (or a cheat-even though this is my no-cheat save) because this is infuriating. Please help. :'(