fawning child's voice, laughing child's voice
哇 centers on the sound of a child's voice: a fawning or laughing vocalization. The meanings are closely tied to vocal expression, specifically a child's playful or coaxing tone.
哇 combines 口 (mouth) with 圭, which likely contributes the sound. The character is used for vocal sounds, especially a child's voice.
The mouth 口 beside 圭 suggests a small mouth making playful sounds. Imagine a child's mouth forming a fawning or laughing voice.
For ア, picture a child opening their mouth wide and saying 'Ah!' in a fawning or laughing tone: Ah -> ア.