Hi,
The others have given you some helpful info. Lilo and Stitch are certainly usually in Stars n Cars Parade in the Studios, which is in the early afternoon ( or around 5pm in summer season ).
If you did not see Stitch Live show in the Studios last time, then your DS will love this, and it is also good fun for adults. ( I was there with my DH at the beginning of the month, and Stitch recognised him as an escaped Space prisoner.....)
This is an interactive show on a screen, where Stitch talks to members of the audience, so most kids love it. He asked kids to make animal noises, and the best ' monkey ' had his photo on the screen.
They do have different shows for this - he doesn't always blow up a planet by mistake!
Stitch and Lilo were both in the show on Central Plaza Stage in summer 2010, but not this year, as the shows change. He also used to appear in Discoveryland at times for a dancing show, but not currently.
You can't guarantee which Characters will appear in the Character Dining restaurants, as they are not the same all the time.
It is worth checking the programme before you go, as this lists the times and locations for meeting certain Characters in both parks.
news.disneylandparis.co.uk/UK/EN/Neutral/Images/ShowsAndParades.pdf
Being Halloween Festival, the current programme features more Villains than usual.
This website gives some idea of where Characters may be found in DLP. According to this, he is randomly in Cafe Mickey, but always in Inventions Character buffet dinner, in the Disneyland Hotel, so you may wish to book there as well.
www.charactercentral.net/RCS3_DisneyCharacters_DisneylandParis_CharacterFindCheatSheet.aspx