A -> B -> C

aaa_srv.erl

-behaviour(gen_server).

init(_) ->
    {ok, #state{member = undefined}}.

handle_call(_, _, #state{member = undefined} = State) -> {noreply, State}.

handle_cast({sync, Pids}, State) -> {noreply, #state{member = Pids}}.

aaa_sup.erl

-behaviour(supervisor).

start_link() ->
    case supervisor:start_link(?MODULE, []) of
        {ok, Pid} ->
            Pids = [Pid || {_, Pid, _, _} <- supervisor:which_children(Pid)],
            ok = lists:foreach(fun(P) -> gen_server:cast(P, {sync, Pids}) end, Pids),
            {ok, Pid};
        Err -> Err
    end.

bbb_srv.erl

-behaviour(gen_server).

init([SupPid]) ->
    ok = gen_server:cast(self(), sync),
    {ok, #state{sup = SupPid}}.

handle_cast(sync, #state{sup = SupPid} = State) ->
    %% この時点で全員の子が取得できるはず.... initの処理が重いとまずそう
    Pids = [Pid || {_, Pid, _, _} <- supervisor:which_children(SupPid)],
    {noreply, State#state{pids = Pids}}.

bbb_sup.erl

-behaviour(supervisor).

start_link() ->
    supervisor:start_link(?MODULE, []).

init(_) ->
    SupPid = self(),
    %% mfa以外は省略
    {ok, {{one_for_all, 5, 10}, [#{start => {M, F, [SupPid]}}, #{start => {M, F, [SupPid]}}, ...]}}.

aaaはtemporaryじゃないと使えない