The Rolling 6 design is an algorithm-based extension of the 3+3 design proposed in 2008. This design has been reported in the literature for testing anticancer agents in children since 2011; it is increasingly used and was reported in 15 (17%) of the published pediatric clinical trials since 2009 - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4818190/

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Note added at 46 mins (2020-06-03 23:46:21 GMT)
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Several other rule-based designs have been proposed, including the isotonic regression model ( 39 ), the biased coin design ( 9 ) and its variations ( 40 , 41 ), and the “rolling six” design ( 42 ). The rolling six design was originally proposed as a way to shorten the timeline of pediatric phase I trials by reducing the number of times a study is suspended to accrual ( 42 ). This method allows accrual of two to six patients concurrently onto a dose level based on the numbers of patients who are currently enrolled and evaluable, who experience a dose-limiting toxicity and who remain at risk of developing a dose-limiting toxicity. Because pediatric trials are typically conducted only after completion of adult phase I trials, this design is intended to shorten the study duration in situations in which there is prior information about the dose range to be evaluated - https://academic.oup.com/jnci/article/101/10/708/969691

In the Rolling 6 design, a total of six children can be enrolled concomitantly at the same dose level (9). The dose level allocated to a new patient is based on the number of patients currently enrolled and evaluated, the number of patients experiencing DLT, and the number of patients whose evaluation is pending at the time of new patient entry.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4818190/