Is there a way in W7 to have all network computers shown the same way as they appear in XP network neighbouring, that is to say to have a tree, each node of which is a domain or a workgroup and lists the computers it contains ?
Unfortunately, the short answer is no, not "the same way".
Windows 7 has gone all Windows Explorer all the time and instead of Network Neighborhood and or My Network Places, you have Network, located bottom left of Windows Explorer. It takes some brief getting use to, but after a while I think you will find it at least as useful as the old way. Like if you change the view to details, you'll be able to see information regarding workgroup membership, location, etc, you can add columns to view additional information as you see fit like discovery method MAC address, IP address, etc.
If all the computers are on the same subnet they should show up here, I have 6 computers Win7 (Ultimate and Pro) and XP pro, with three different work groups, they are all listed and all accessable through this interface.
Domain Name Resolution is handled by DNS so, Check the XP machine(s) my guess would be that they are either pointing to a different DNS server, or a second DNS server or that the name resolution is being augmented by a WINS server that your Win7 machines aren't looking at, or perhaps even have a hosts file with the machines in the child domain.
If there is nothing unique or unusual about the machines in the child domain and the Win7 machines are pointing at the DNS server responsible for hosting the A records for that domain, then I'm not sure why you're not seeing them.
Perhaps make sure that NetBIOS over TCP/IP is enabled as well as starting the TCP NetBIOS helper service and see if that makes any difference.
EDIT: Can you ping the machines in the child domain by ipaddress and hostname from the Windows 7 machines.
Both the XP and W7 machines use the same DHCP server, which is the root domain DC. So, apart from their IP address, their NIC have the same configuration and points to the same DNS and WINS. So, I expect the network be seen the same way from either machine. There are two DNS servers, the root domain DC and the child domain DC. The child redirects to the root and the root delegates to the child.
From the W7 machine I can ping the child domain DC by IP address, by Fully Qualified name, but not by the machine name.
From the XP machine I can ping the child domain DC by IP address, by Fully Qualified name, and by the machine name.
Why W7 does show root domain machines IP and MAC addresses ?