alternative is u2fzero, available on amazon for 8$, and totally open source. the difference is that yubi uses an nxp secure coprocessor, whereas the u2fzero uses atmel. there is the possibility of side-channel attacks on the u2fzero.
but for your family, it is better than nothing and much more cost effective.
u2fzero is "Currently unavailable" on amazon. And the lack of housing makes me question how durable the device would be. The last thing I want is my u2f dying and locking me out of a ton of accounts.
The Atmel chips do not claim that they have implemented counter-measures for power analysis, etc. Power analysis on a key operation is a dangerous attack if proper counter-measures are not taken. You can literally read off the 0/1 of each bit of the key as the key operation is underway if you monitor the power/timing.
The NXP chips inside the yubikey claim to be hardened against several such attacks (although I have not confirmed).
NXP is a cagey company. For example, I am a researcher, and I wanted to get the yubi-key's unlocked to write and test new u2f protocols on their hardware. They wouldnt sell me development keys, and claimed that the restriction was placed on them by NXP. I wrote half-a-dozen requests to the NXP people, and they never replied.
but for your family, it is better than nothing and much more cost effective.