First off welcome.

Secondly, I was under the impression the mark 3 Sirion (box shaped) had remote central locking as standard? The previous mark 2 produced from 2001-2005 had this as standard.
From the sounds of it the top of the range mark 3 Sirion which your daughter owns e.g the 'SX' should definitely have this already fitted. I remember an owner on here who had a 2006/7 model had this as standard and I don't think it was an SX model.
Worth checking the owners manual first before going digging into the car electrics as you maybe adding something extra you don't need.
As for the remote fobs, then these are standard option remote fobs which span many a Daihatsu model. Saying that I have seen second hand remote fobs on eBay starti g at 99P! But technically you should have got this when you purchased the car.
Hopefully someone who owns a mark 3 Sirion comes along to confirm this

Pictures of the Daihatsu fobs:

http://www.daihatsu-drivers.co.uk/node/17508

http://www.daihatsu-drivers.co.uk/node/17509

Normally 2 fobs can be programmed

Edited to add:

Just done a search on auto trader, and all models of the Sirion mark 3 are advertised as having remote central locking as standard. There was even a picture of a 2007 SE model (One below the SX) with the keys and the Daihatsu remote fob attached to the keychain such as the one pictured in the 2 links above. Definitely worth seeing about getting ho,d o a replacement fob. Bu be warned the dealership will want a small fortune. Bu maybe a member on here may have a spare one for sale definitely a better option than wiring in another nit, unless you remove the original and fit this 3rd party untested unit