The current gift free threshold to a niece or nephew is €32500. This is a life time limit. This is for both gifts and inheritance for anyone in category B ( blood relatives aunt/ uncles , nieces/ nephew, brothers / sisters, grandparents.. not parents.. this is defined in tax law)
The small gift exemption of €3,000 is annual gift exemption and is available from anyone.. so you could get say 10 gifts of €3k and your tax free thresholds would be unaffected and all the cash would be tax free.
Assuming that your niece and nephew never got a gift exceeding € 3 k from any of those blood relatives and have never got an inheritance for those blood relatives either .. then the gift will be tax free and they will have approx €2,500 tax free threshold remaining (33k less 3k= 30k less 32,500= 2,500 of tax free threshold in cat. B. ).
As the niece and nephew will have used over 80% of their threshold after gift then they must file a CAT return.
Don’t forget you may have a CGT liability yourself on the disposal of the property. The deemed proceeds is the market value of the property..