Is it true adults are at reduced risk of appendix bursts because of the fat which contains the infection? but children don't have as much fat?

Could be, but...: Multiple factors decide how a disease may behave and progress, and more tissue such as fat around an abscess like fulminating appendicitis is likely to have more local tissue to wall it around, but such condition should not be construed as the sole factor to predict how it may do clinically. So, what you mentioned would not pose any clinically significant impact on how a patient should be managed.