Many parents get annoyed at their children playing with cubes because they can be quite loud, especially some modern 3x3x3 speedcubes. However, some are quieter than others.

Quietest 3x3x3s:

The quietest regular sized 3x3x3 on the market is usually regarded to be the DaYan TengYun V1, this is still enjoyed by a lot of speedcubers to this day but is a bit unstable and does pop more easily than other modern speedcubes.

DaYan released an updated TengYun V2M which features magnet adjustment and a slightly better anti-pop mechanism, it is still quite quiet but not as quiet as the V1, in general DaYan cubes have been quieter than other brands recently.

some QiYi cubes are also rather quiet such as the Tornado V3, the tornado v3 is a very nice speedcube and is very smooth and quite quiet, it was also used by Max Park in his 3.13 second world record single.

Another option to consider is larger cubes, big cubes are generally quieter than smaller cubes despite there being more pieces. If you get your child a DianSheng big cube then they are likely to take a long time quietly solving it and you will hardly notice!

Ultimately cubes are going to make a noise and it is something that people around cubers have to learn to deal with, if you want to completely eliminate the noise from a cube then you will have to get the cubelab 1cm 3x3x3!

The UK cube association delegates have actually banned me from bringing my DianSheng Googol 35cm 3x3x3 to most UK competitions as it is too loud! And it is pretty loud.