Every child will have a computer, and possibly a tablet. However, they may not always be home. There might be a file they will need to access that is on their home NAS. If this is the case, they will normally not have access to that file. However, when using a VPN, they will just need an internet connection. The computer will then connect to the VPN server at home, and log them in. From there, they will be able to access their NAS, and any other resources such as the printer. Even though they are connected to the VPN, they will still be able to do things like web browsing, or email. Everything will just be a slightly bit slower, and since I will be seeking a 2GBPS FiOS connection, the connection should be faster than normal. The delay should be absolutely minimal. The big benefit is access to their NAS.
Another benefit is if they have the Tablo app installed on their tablet, or phone – they will be able to watch TV that is stored on their Tablo. This should not be done during school time, but it can be done during breaks, or after school hours. Tablos are usually not available outside the LAN, but with the VPN, the computer is treated as if it was a part of the LAN.
If the foster child was no longer under my guardianship or care, their VPN credentials will be deleted, and they will no longer have indirect access to the LAN. If they were to visit, they will be able to access the guest network, but this will not give any access to the network other than internet access.