Children macro uses large amounts of memory with a large space

Confluence.atlassian.com had memory problems where the heap allocation needed to be increased from normal 512 MB up to 1024 MB to not die with OutOfMemoryErrors. According to Donna:

"Charles was able to locate the problem. There was a page that was using the children macro. Unfortunately there were 2000+ children. This was getting sucked into every recently-updated RSS feed. Not pretty."

We should profile the children macro and reduce its memory usage, at least on subsequent calls which return the same result.