wyres
Grandson is wyr, granddaughter is wyres. Grandsons is wyrion, granddaughters is wyresau. There is not a neutral form of grandchild. If you wish to refer to multiple grandchildren, some of whom are grandsons, use wyrion.
The Hebrew word for granddaughter is nechda - נכדה
The Welsh word for Toxic is: Gwenwynig
The word 'fern' is not a Welsh word and has no meaning. The Welsh word for 'a fern' is 'rhedynen'.
The Welsh word for mum is mam.The Welsh word for dad is dad.
The Welsh word for Excellent is ardderchog, it is also the welsh for fantastic.
The Welsh word for 'decibel' is desibel.
The Welsh word for planet is 'planed' (plural - planedau)
The Welsh word for thunder is "taranu."
The Welsh word for 'rectangle' is petryal.
The Welsh word for 'a cave' is ogof.
The Welsh word for 'landowner' is 'tirfeddiannwr'.