dyzzypyxxy said:... you'd be wise to give them some little sticks to support them until the stems get stronger.
I was thinking "maybe cover them with clear plastic before heavy rains", but if that were easy to do, the OP probably already would have.
Funny, we all know that "hardening off" lets seedlings gradually develop tolerance for drying wind and hot sun. But it also lets seedlings develop enough stem-strength to resist big rain drops. Maybe the key phrase is "until the stems get stronger".
Some people put an intermittent, small fan on trays of seedlings to exercise their stems. Others "beat them up" by brushing the seedlings with their hand.
I think the "best exercise" for seedling stems is bright, bright light while they are growing, before you ever put them outside. A shorty, stocky seedling can take a lot of abuse that would topple a tall, leggy seedling no matter how protected it is.