To stuff a heart into a round I do just like Jillybilly said. No special tool is required. Some people even use the ends of their glasses.
I use the wooden end of a paint brush and stuff the heart into the round.
When making the confetti wands (putting cupcake sprinkles into clear heart or rounds) you can take a tiny funnel if you want to be neat or just carefully pour the sprinkles into the clears. I do this over the kitchen sink with a glass measuring cup inside so that I can save what I spill. After filling a bunch of them up I take scissors and cut the nozzles off some old large rounds to work as rubber bands or you can use actual rubber bands to group about 5 or so together at a time. This also keeps the sprinkles loaded into the balloons. I then put these into a baggie that I carry to keep the sprinkles from making a mess.
To stuff the tulle, bouncy balls, or other stuff in balloons - meet my little friend!
The Elastrator, veterinary tool, works wonders for stuffing things into balloons. These can be purchased new off of ebay for around 10 donuts and this includes shipping.