The first few times your child and your girlfriend's are invited to play together in the same space, they may not find them and there may be whining from now on. What can you do in this case?
Ideally, the child should have the right not to play with our friend's child, but this is practically impossible for obvious reasons. So a solution of necessity would be to meet sometimes with your girlfriend outside the house and leave the children home with their dads.
At the second level, you should definitely listen to what the child has to say about this refusal and not just try to force the other child to tolerate for a few hours.
In some cases the negativity may be pointless, but in others it may have a specific background.
As fair as the reasons he gives you are, politely inform him that sometimes he will have to retreat and spend time with the specific child of your girlfriend and you in turn will say it in peace. A simple example that your little one will be taught the concept of compromise, which is necessary sometimes in life.
Based on what he will tell you, you will be able to explain it to someone in a different light, which due to age can not be imagined. What does this mean in practice?
That some behaviors that bother him may be misunderstandings.
Encourage your little one to give time to himself and the other child to get to know each other better and maybe bridge some (if not all) of his differences. Hasty conclusions are not good advice!
Finally, remind your girlfriend of the situation so that she can have a conversation with her little one so that she does not find out that something is wrong with your child's life.
Source: Newsbeast