The only way that you can really ensure your children can attend a Piedmont school would be to move to Piedmont.
Have you researched the elementary chools in Oakland? There are some really wonderful schools in Oakland. I have a son in an Oakland public school and I am very happy with where he is. I also have several friends with kids in different Oakland public elementary schools who are very happy with their school as well.
You may want to start doing so reasearch into schools in your area. You can ask friend and neighbors, or there area some online resources. If you don't already know about it the UCB Parent Network is a great resource to ask about schools http://www.parents.berkeley.edu.
Also you can check out http://www.greatschools.net.
Also as an Oakland resident you can apply for what is called and inter-district transfer if you find a school you like that is not in your immediate neighborhood. If you have more questions about this process, please feel free to contact me directly.
If ultimately you decide that Piedmont is still where you would like to have your child attend school, you will need to consider moving to the Piedmont school district.
Good luck to you on your school search. I know it can be a stressful time when determining the the best school for child and family.