Try spraying them with a dish soap solution. A few times a day should do it.

Or water from your hose sprayed a bit forcibly. Easy-peasy.

Linda

Here's a handy link for you:

http://www.growingagreenerworld.com/aphids-how-can-they-be-controlled/
Since your seedlings are so tiny you may not want to spray with too much force.

I remove aphids from the milkweed by regularly checking the growth tips for them. This is where they start to multiply, guess they love the tender foliage. Just gently brush/rub them off with my fingers which prevents them from colonizing to the rest of the plant and it really does seem to work.

Maybe you could use a paint brush or basting brush and brush them off gently.

PS I have pretty much gotten over the "squeamish factor" of squishing aphids and beetles, etc.