Booster seats

So I have a 4 year old step daughter and a 1 year old son. We just currently moved and the way the new kitchen is set up my sons high chair won’t fit. The 4 year old has had a booster seat for ever and we just figured we’ll get my son one. But I don’t have money right now for one, so when we have breakfast together I put my son in the booster seat. And the 4 year old always throws a fit saying it’s hers. What are some good ways to explain that she’s gotta share the chair right now. And what are some good booster seats. I really need one that I can buckle my son in. He thinks standing on the table is funny 😒