Hi Daniele,
you could also look at Psyscope X. It is freeware.
http://psy.ck.sissa.it/
It's Mac-only, but has good timing accuracy and fairly easy to use. 
They also have a very responsive user group. I should say though - 
it's not COMPLETELY straightforward to ensure very very precise timing 
- but there is a lot of info on the website for sound timings. You 
could play sounds, and collect all keypresses time-locked to the start 
of the sounds.
For analysis. you could use the output (key-press times) from Psyscope 
and use these to markup the sound files using Praat 
(http://www.fon.hum.uva.nl/praat/) which is also freeware.
Mo
On Mar 24, 2011, at 4:32 AM, daniele schon wrote:
Dear all,
I am trying to think to a simple and cheap solution to record 
children in a tapping task (eg along with a metronome or a piece of 
music).
I have up to now found several solutions commonly used, but none of 
them is cheap. For instance, using a software such as Presentation to 
send the auditory stimulus and record with the space bar (requires 
Presentation/E-Prime license), or using Cakewalk and a tapping pad 
(requires Cakewalk license and a tapping pad).
So I would be glad if you could share your advice on the "optimal" 
solution, optimizing the real cost (in euros), the precision (in 
milliseconds) and the other cost (the hours I'll spend analyzing 
data, for instance recording the tapping on a microphone ...).
my warmest regards
daniele
--
Daniele Schön
Equipe Langage, Musique et Motricité
Institut de Neurosciences Cognitives de la Méditerranée
C.N.R.S, 31, Chemin Joseph-Aiguier
13402 Marseille Cedex 20, France
Tel: 0033 (0) 4 91 16 41 30
Fax: 0033 (0) 4 91 16 44 98
http://www.incm.cnrs-mrs.fr/pperso/danieleschon.php
I CHOOSE PEACE