Doing "each letter separately" would be transliteration (or transcription), not translation. It is an option, but it would not make the name "Elvish", only written in another alphabet (i.e. Tengwar, the Elvish writing system). And I would not recommend translating a name to the Elvish languages for a beginner. Both Quenya and Sindarin are too complex for that to go well in a first attempt, especially with Sindarin (and the 140-or-so phonetic rules that govern it). Luckily, I can help.
The meanings that you mentioned for Kathryn and Maisy seem to be correct, but I rather found the meanings "man" and "army, warrior" for Charles. Assuming that you want these to be translated to Sindarin, I offer:
Charles - Belvaethor "Strong Warrior"
Kathryn - Glandis "Pure-woman" (with glân being "white, clear, bright, pure")
Maisy - Aerviriel "Sea-jewel-daughter" (there is no attested "pearl" word, thus "sea-jewel")
And this is how they would be written in Tengwar.