The problem when multiple PCs are active might be down to the speed of the internet link (try visiting www.speedtest.net - there are systems pretty much worldwide so there should be some near you - which country are you in, by the way?)
Alternatively, depending on the wireless protocols supported by the router (and what they are trying to do) then it might be a simple adjustment.
If you can access the control panel of the wireless router, you can check the connection speed between the router and the exchange down that phone line. Higher the better, but be warned the data coming in via your ISP might be at a significantly lower speed than the link to the exchange.
You can also check the Wireless settings. It is likely the router supports 802.11a and 802.11b (low speeds, up to 11 Mbps), and probably 802.11g (up to 54 Mbps).
Then at the laptop end, they will likely support 802.11b and 802.11g but it will depend on local rules exactly what settings are allowed and 'common'.
If your youngsters are watching videos on YouTube, for example, they will both put quite a lot of traffic through the wireless link, and the speed of the link should cope but there can be situations where one or both might pause and they will get the dreaded 'buffering' notification.