To be honest, I'm not all that familiar w/ PCI compliance standards. But based solely on your own description, I assume at least part of that compliance requires keeping the POS system(s) inaccessible over wireless.
I'll make some reasonable assumptions, but please correct me if those assumptions are wrong.
[modem](lan)<-- wire -->(wan)[wired router (192.168.1.x network)](lan)<-- wire -->(wan)[wireless router (192.168.2.x network)]
Let's assume as well that the wireless router has a LAN IP of 192.168.2.1, and a WAN IP of 192.168.1.2 (from the wired network). So clients of the wireless network receive IP addresses in the 192.168.2.x range, and have a default gateway of 192.168.2.1.
Given the above, there's no reason that wireless clients shouldn't be able to access the wired clients/devices of the wired network. The wireless router is aware of the location of the 192.168.1.x network (since it exists on its WAN port), therefore any network requests by clients of the wireless network should be directed to their gateway (192.168.2.1) and forwarded to the wired network. In fact, it’s so easy, I have to wonder whether this actually meets PCI compliance standards!
If this isn't working, then perhaps my assumptions are incorrect. Sometimes ppl make the mistake of not using different subnets when connecting to adjoining networks. That's a mistake and should be corrected.
Looking at it from the other direction, the wireless router is doing its job by making its network inaccessible thanks to its firewall. In this case, I'm not sure you need the firewall on the wireless router since I don't know exactly what you're trying to accomplish w/ this network configuration. As it stands, both the wired and wireless networks are already protected from Internet threats by the wired router's firewall. Only you can decide if the second firewall between your local networks is necessary. If you drop that firewall, or at least open the necessary ports for the services you need, then you can add routing information to the clients of the wired network so they can locate clients/devices of the wireless network.
route add 192.168.2.0 mask 255.255.255.0 192.168.1.2
The above says "you can locate any 192.168.2.x address by using the gateway at 192.168.1.2".
You can either add the route to the individual clients, or if your router supports it, add a static route to the router's routing table so that wired clients are automatically rerouted by the wired router to the wireless router.
All that said, realize we’re only talking about explicit IP addressing here. You will still have problems using *named* resources since they exist on different ethernet networks, and therefore don’t broadcast between them. I suppose to some extent this meets your PCI compliance standards. But that’s not really any sort of meaningful protection since all clients/devices are addressable via explicit IP addressing.
If you want to add *named* resource capabilities between the networks, then you either have to use a commonly shared WINS server, or else a per-client hosts file, that maps IP addresses to named resources.