We visited during Easter holidays (4 of us including children of 6 & 3).
The gardens are really enjoyable but are quite small. We had lunch there but stayed for little over an hour.
There's a huge variety of cactii and it's really well laid out and maintained. Entry fee is pretty reasonable (most attractions are in Lanzarote) but food & drinks quite expensive.
We were staying at Playa Blanca and visited on our last day on a circuitous route to the airport. It would have been slightly disappointing if we had travelled so far just to visit the gardens.
I'd suggest that the Cactus gardens is best combined with a trip elsewhere (eg Jameos del Agua).
Nearly forgot - there's a windmill here but there's no window so not really worth the effort of getting small children up & down the stairs.
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