Here is the evidence: https://imgur.com/a/3Xr9pJq
This isn't evidence that you invested in Bitcoin, bur rather that you have $83.40 of something. Maybe even US dollars?! If I had to guess I'd say you bought Ethereum not Bitcoin as the inferface looks like MetaMask. Furthermore, there is the options for tokens/nfts (which Bitcoin wallets don't provide). Sorry for being the sceptic here, but if you're going to provide "evidence" of BTC then it should a sat-based amount.
Ultimately though, it sounds like nobody really cares whether you bought $50 for your baby or not. Unless the UTXO has been time-locked for a later date, there's no evidence to prove it for the little one either
It's not a Metamask wallet, although I no longer use Trustwallet but I guess he is using it and Trust is a multi-chain wallet that includes bitcoins. When you use Trustwallet, it will show your total assets in USDT unless you go into the details of each coin, it will show sat.
But in the end, it's OP's right to buy bitcoin; he doesn't need to prove it to anyone. If he invests in it, then he will benefit from it, and vice versa if he does not invest in bitcoin, it is still his decision. No one will care about that.