I'm not entirely certain where you are coming from
Is WiFi more vulnerable to intrusion. - Of course it is! - the whole idea is to broadcast - and few people have lead lined walls
a piece of wire is difficult to intercept undetected.
Is your daughter at risk ... of course she is - esp if she surfs in her room with the door closed ... but that's the internet - there isn't an added threat from WiFi ..... unless she banks online ....
are you more vulnerable .... of course you are.
anyone with a little knowledge can break wep wap wup wibble or whatever key you decide to use ...
my personal record is 42 seconds - average is 20 mins on a wap protected network (setup at work ... it's that or bubble bobble at break!)
(the spotty little oiks are called road warriors.... they tour the streets in their mums' fiesta armed with ... a laptop (and often a GPS) easy signals are mapped and logged onto online databases so others can visit.
(see netstumbler
http://netstumbler.com/)
is it likely to happen? .... actually probably not very ... if you take reasonable precautions .... you are probably more likely to get your fence "tagged".