Ouch. That’s not a very nice thing to hear. My LO’s joint account is with me, but my in laws know I won’t touch the money, so they just credit money into that account when they want to give my LO money. If they really don’t trust you, then you can suggest to your MIL that her son and your LO can make the joint account, or she can wait till your LO is an adult, before she gives the money.