Rather than totally blame the Apple device you might consider some of the following...
1. Its possible your wireless firewall/router is a bit faulty. Over the years of working with computers I have seen a number of these devices. Manufacturers tend to cram a lot of heat producing electronics into tight sleek looking plastic cases with an eye toward making them attractive to consumers instead of making them durable. Over time the heat produced by the device tends to cause the electronics to start getting flakey. While it might work marginally well with one or two users its possible to see more issues as the number of users increase. Touch it. Does it seem unusually warm or even hot? If so consider replacing it.
2. Basically what all wireless firewall/routers are is small computers. Inside that plastic case is a embedded operating system usually running on some form of Linux or BSD. Most consumers who use these devices are unaware that these devices do occasionally need updates. If you know how to log into the administrative interface of your device you will find a section devoted to installing what are called firmware updates. Its possible your particular device suffers from a bug that was corrected by the manufacturer after you purchased it. This is also important to check for security purposes since some updates correct missed security issues in the device. How this is done varies from brand/model. Post your brand/model and I can point you in the right direction.
3. Its possible your granddaughters phone is downloading an excessive amount of data. Perhaps multimedia, movies, music, streaming etc. Perhaps your wireless firewall/router is an older one with slower wireless speeds.
More info would help.
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