The Parent and Child Rasengan is cooperation ninjutsu whereby two shinobi, most often a user and their child as the namesake suggests, combine their Rasengan into a massive sphere, which is then thrust into their target.
Minato and Narutos Rasengan
Naruto and Minato, in their Tailed Beast Modes and with Naruto in Sage Mode as well, collaboratively create a Rasengan of their chakra avatars' size, then slam it into the target. This attack was strong enough to push back Obito Uchiha as the Ten-Tails' jinchūriki.
Naruto and Borutos Rasengan
Boruto starts off by generating a tiny Rasengan. Clasping Boruto's hand, Naruto then adds his own chakra to the Rasengan, forming a gargantuan sphere which dwarfs both in size. Boruto used this technique to kill Momoshiki Ōtsutsuki, himself deemed a threat greater than Kaguya Ōtsutsuki by Sasuke Uchiha, on top of obliterating Momoshiki's Planet's Shinju and penetrating the perpetual cloud layer on the planet.
1. Though clearly not being a normal Rasengan, it was simply called Rasengan in the manga as well as in Boruto: Naruto the Movie. 2. In the novelisation of Boruto: Naruto the Movie, the Rasengan that Naruto and Boruto created functioned the same as Boruto's Vanishing Rasengan on a more powerful scale.