Well in Pa your children do not need to start formal schooling until they are 8. This usually means third grade. And I believe this is in effect as soon as you officially move here. Now if you are just visiting your parents and not living here that is one thing, but if you are permanently moving here, then you would need to start them. I wish I knew the legal terms for you. I don't know if the state would consider it visiting since you are not living in a residence that you own. But anyway if they do consider you legal residence once you get her, your 4 year old you don't have to worry about. I am homeschooling my 5 yo and don't do anything official. I won't need to do anything official until she is 8. And I can also start her in public school at anytime if i decide to. She will be 6 next summer and I could send her to first grade next fall, or wait another year and start her in second grade, or even wait until 3rd grade to start her, or at that time officially homeschool her. It is the 8 year old you will have to do something with. You could enroll the 8 year old in charter school/cyber school, which is a public school where you teach at home, but they supply everything you need. It is free since it is a public school. You could also officially enroll in homeschool, for the 8 yo but it sounds like you didn't want to do that. The other options I believe are all illegal. Which would be to just keep the 8yo home and teach at home without officially homeschooling, or send them to the desired school using your address on the house you own. I don't believe legally you can do that, I believe you need to actually reside at the address and not just own the house. But you could check that with the school district, letting them know you own the house and will be moving there once the tenants vacate. It is worth a shot if you really want to start the 8 yo to start right away, otherwise i would recommend just enrolling the 8 yo in a cyber school, you could do at home wherever you live and then once you are settled in your permanent home, enroll one or both at the desired school.
Edited to add: I just did a bit of research and what i found so far is that in PA you need to be a legal resident in order to send your child to school and you need to show proof of that, things such as household bills qualify. If you do not yet have a permanent address, you are not considered a legal resident, so you may be able to avoid sending the 8 yo to school with that. However I would call the call the school district you want them to go to, and say we will be moving here can we start the child/ren while visiting relatives while we find housing, and if they say no, ask what you should do as far as schooling goes since you will not be residents in your parents house either, they should know the legal answer.