Kids are tricky (and costly). In Czech on top of it is a child of neutre gender but children in plural are feminine gender.... Thus VELKÉ
It's because é pronounced as doubled e, as with every ''gachec(?)''vowel sound
Yes, it's pronounced longer, however, it is not called "gachec" but "čárka" - I am speaking about this little comma written over a letter: " ´ " so you get vowels like á, é, í, ó, ú, ů, ý, all of which are pronounced longer (á like aa).
By gachec you probably meant háček " ˇ " which can also be written over come consonants or ě so you get letters like ě,š,č,ř,ž,ň,ď,ť - their pronunciation is different from the letters without háček over them. E.g. ň is like Spanish ñ, š is like English sh, č is like English ch in the word check, etc.