Locking pokecoins

hello I let my 4 year old play on my phone to play Pokémon go while at walmart. that being said yesterday he spent 400 earned pokecoins (i have a lock so he cant spend real money on anything) but is there a way to put a pass code on spending pokecoins. may be controversial to let my kid on my phone but I know pokemon is a safe game and he cant talk to people or watch anything inappropriate so please no comments like "just dont let him play" He is very into pokemon and me and my husband both really love it. another option we considered is maybe a ds or Gameboy to let him pkay but then I dont get all the extra pokeballs lol