You can use the pipe character in a URL for reaching child attributes on attributes, so following is perfectly valid:
Where "Above Average" is a child attribute for "Pressure" on the element "Boiler". You can also use this same syntax when using Batch queries in the Web API. If you have a specific example of your batch query not working please feel free to post it and we can take a look at the syntax. If you do need to escape characters, the list of characters that do need to be escaped can be found in the help documentation along with their associated URL encodings.