Auto investing in ETF from RESP - Questrade account

Your plan is quite reasonable.

Aren't you lucky. There's more than one way to address this in Questrade, even if not ideally.
A page on Questrade site mentions the 2nd way, as well as managed portfolios (which I assume you won't be interested in, as they come with an extra 0.25% management fee).

1. Both XGRO and XEQT are supported by BlackRock PACC plan: Blackrock can take money directly from your bank account and handle share purchases. Only whole shares though, which means it will be typically less than $200 total, and you may need to top up the difference once in a while manually. You'd need to fill a PACC form and mail to Questrade to finish their portion.
2. Alternatively, use Passiv service (Elite level), which is free for Questrade account holders. Depositing money from the bank to RESP would be 1st (automated) step. Actual share purchase with Passiv does not sound like fully automated; they say "1 click". Still, should help.