Nia

Hi! I am new to this forum and im finding it quite interesting!
I have a question. My dog is 6 weeks into pregnancy. she's about 2 1/2. We've set up her whelping area but we have 2 other dogs as well. One is 6 months and the other is 3 years ld. The smaller one keeps coming in her whelping area. How do i get my female to get used to her area and should I keep the little one away?