I'm sorry your girl has issues with other basenjis, we have a weekly meet up in Tampa, and I know of another one in the Palm Beach area. All our basenjis are friendly and very social and do well with new basenjis, even cranky ones (my newest adoptee is a bi**h). Three dogs is actually a good number. Your dogs are still young and likely will be more tolerant of a puppy now, than if you wait till they are old and cranky. And puppies do have "puppy license" for about 6 months when he adults are very tolerant.
As for kids, no personal experience but several friends have had to work through issues when their baby became a toddler and was moving quickly, grabbing, etc. Their sweetest dog had to resort to growling and even snapping and nipping the 'aggressive' toddler, one friend even wanted to rehome one of her dogs…fortunately they kept him and a year later things are much better. Similar situation with another friend right now as their baby is crawling and grabbing, it is her most easygoing dog who feels threatened by this wild baby. A dear friend raised 2 boys with basenjis.
You can't always predict how they will react, you just have to be calm and ready to do a lot of separation for many months till the human kid is able to learn how to relate to the dog. You protect each from the other for awhile!
I once worked with a rescue person who wouldn't let a dog go to either a lawyer (they worked for lawyers and felt they were 'too busy' and didn't have time for dogs) or a young couple who will be starting a family (many basenjis end up in rescue due to this rough adjustment period when the baby is between 6 months and 2-3 years) so you would be a double strike out for her, haha.
It's good that you are looking at all eventualities, but it is possible to overthink this issue. Some dogs are more jealous of a baby than others, some more afraid, and hard to know in advance. Be prepared for at least one stressful year (as if having a new baby isn't stressful enough!). Good socialization and ongoing 'training' keep the dogs aware that you are the leader, basenjis who think they are the boss are much harder to manage, and schedule plenty of dog time after the baby comes.
Good luck to you! If you are in the Tampa area and want to give meet and greet another chance, just let me know.