Summary of this excellent article
Normal appendix
blind-ending, tubular, and nonperistalsing segment of bowel
arising from cecum
compressible
no surrounding mesenteric edema

Appendicitis
dilated
non-compressible
Doppler for hyperemia
periappendiceal fluid
periappendiceal inflammation (echogenic change): reliable indicator of appendicitis

Diagnosis of appendicitis 
Probe: curved then linear for better resolution and compression
Diameter cutoff: wall-to-wall 6 mm
(7 mm for no secondary signs of inflammation, such as lymphoid hyperplasia or fecal impaction of appendix)
Graded compression: non-compressible
(Pitfall: muscle guarding, appendix slide away, operator technique, patient obesity and transducers)

Sonography for Appendiceal perforation: Sensitivity 86%
Findings:
Echogenic submucosal disruption
Complex periappendiceal fluid
Abscess

Benefit of sonography as first line
quick
inexpensive
available
reliable in skilled hands
no ionizing radiation exposure

Mesenteric lymphadenitis 
the most common appendicitis mimic
Lymph node size: > 10 mm
Doppler: hyperremia
Compression: tender

Intussusception
the most common cause of bowel obstruction for age < 5 years
triad (<40%): colicky abdominal pain, palpable mass, bloody stool
sonography: imaging modality of choice
Sensitivity & Specificity: about 98%
Target or pseudokidney signs (echogenic mucosa-hypoechoic muscularis-echogenic serosa)
Sandwich signs: outer intussuscipiens & inner telescoping intussusceptum
Size:
< 1.5 cm (small bowel, ileoileal , most resolved spontaneously)
> 2.5 cm (fatty core or lymph nodes, need emergent reduction)
If presence with diffuse small bowel wall thickening
==> consider lymphoma or Henoch-Schonlein Purpura
Role of echo: diagnosis & hydrostatic reduction

Meckel diverticulum
the most common congenital GI tract anomaly
related to intussusception, bleeding or diverticulitis (appendicitis mimic)
Nuclear medicine Meckel scan: Sens 81%; Speci 96%

Ovarian torsion
most common on right side (appendicitis mimic)
Size: most important factor
General rule: unlikely for smaller than 5 cm 
Doppler flow is not reliable

Sonography of Abdominal Pain in Children

Appendicitis and Its Common Mimics ( by TR Sanchez et al.)