app.R

library(shiny)

source("parent_module.R")
source("child_module.R")

ui <- fluidPage(
    parent_mod_ui("parent")
)

server <- function(input, output) {
    callModule(parent_mod_srv, "parent")
}

shinyApp(ui = ui, server = server)

child_module.R

child_mod_ui <- function(id) {
  ns <- NS(id)
  tagList(
    sliderInput(ns("the_slider"), "SLIDE", min = 0, max = 10, value = 3)
  )
}

child_mod_srv <- function(input, output, session) {
  ns <- session$sn
  
  observeEvent(input$the_slider, {
    if (input$the_slider > 5) shinyjs::disable("parent-toggle_disable")
  })
  
}

parent_module.R

parent_mod_ui <- function(id) {
  ns <- NS(id)
  actionButton(ns("click_me"), "CLICK ME")
}

parent_mod_srv <- function(input, output, session) {
  ns <- session$ns
  
  observeEvent(input$click_me, {
      showModal(
        modalDialog(
        title = "This is a modal",
        footer = actionButton(ns("toggle_disable"), "CLOSE MODAL"),
        child_mod_ui(ns("child"))
      )
    )
  })
  
  observeEvent(input$click_me, {
    callModule(child_mod_srv, "child")
  })
  
  observeEvent(input$toggle_disable, {
    removeModal()
  })
  
}