I think she is being a little selfish. Maybe you could ask her if she could switch 2 of her nights a month to staying Saturday nights and taking your son with her, once you meet than man and he meets your approval. Let her know exactly, word for word, write it down if you have to. A lot of being poly is communication and even if it seems petty or small and they think you are being irrational those feelings matter.
As for being discouraged on OKCupid, it is a fairly common complaint. It took a few weeks for people to talk with my husband too. We seriously got lucky in our situation. Try other dating sites as well. Don't worry, it will get easier.
Since knowledge is infinite, it has infinitely fell on me.