Are there oracles for knowing in real time the duration of the Cardano time slot in real time ? One learns in the Plutus Pioneer Program that the duration of this time slot can change, is there a way to give Ada to my children in 3 years (and not 6 or 1.5 years ^^) ?
Simple timelocking scripts have been around since before smart contracts were launched, though they do rely on slot length as their notion of time. You can read more about them at IOHK's simple scripts repo.
While slot length can indeed be changed, that doesn't mean that it will be changed without a significant heads up. Currently, slotLength is a non-updateable protocol parameter, meaning it would require at least an HFC to change, which we would be aware of ahead of time.
For this reason I would argue the slotLength parameter is currently, by far, a more reliable source of time than any oracle. The former has roughly the same security/reliability guarantees as something like Max-ADA-supply, whereas the latter is only as trustworthy as the underlying data aggregator.
If I were time-locking UTXO's for years ahead, I would use the slot-based time locking scripts mentioned above, in combination with an emergency fallover key that is non-timelocked and buried/kept somewhere safe, just in case of whatever.