There's no problem even if you don't book a ticket of him, Ticket checker won't ask any ID proof of your kid to verify his age. Just say he's under 5.
please wait...Translate to HindiKoi dikkat nahi hai agar tum uske ticket book na karo, Ticket checker tumhare bacche ke umr ko jaanne ke liye koi ID proof maangne waale nahi hai. Sirf keh do ki woh 5 saal se kam ka hai.