library(magrittr)

nested <- '
[
  {
    "a": [
      {
        "b": 1,
        "c": 2
      }
    ]
  }
]
'

hierarchical <- '
[
  {
    "a":
      {
        "b": 1,
        "c": 2
      }
  }
]
'

jsonlite::fromJSON(nested)
#>      a
#> 1 1, 2
jsonlite::fromJSON(nested) %>% str()
#> 'data.frame':    1 obs. of  1 variable:
#>  $ a:List of 1
#>   ..$ :'data.frame': 1 obs. of  2 variables:
#>   .. ..$ b: int 1
#>   .. ..$ c: int 2
jsonlite::fromJSON(hierarchical)
#>   a.b a.c
#> 1   1   2
jsonlite::fromJSON(hierarchical) %>% str()
#> 'data.frame':    1 obs. of  1 variable:
#>  $ a:'data.frame':   1 obs. of  2 variables:
#>   ..$ b: int 1
#>   ..$ c: int 2

^{Created on 2018-09-25 by the reprex package (v0.2.1)}