?maybe pokeweed (Phytolacca americana)
and
japanese knotweed (Fallopia japonica)
??

Probably milkweed. The milky white substance is poisonous so be careful around kids. Don't let them get any in their eyes.
Milkweed is the host plant for monarch butterflies. They grow quick and shoot up, then bloom beautiful pink flowers. The monarch butterfly lays the eggs on them adn the caterpillars only eat milkweed. They have a DEEP taproot. Was this a butterfly garden before?
You can do some great education with this.
Much milkweed has been eradicated across the country because it was a weed and monarchs are struggling to survive. Hundreds of people across the country are planting and growing it to assist the monarch from extinction. Google it and you'll find all kinds of articles.
They'll get 4-5 feet tall.In the fall the milkweed seed pods are fabulous. They start green, about the size of 2 golf balls and slowly turn brown. If you leave them they split open and the seeds are carried by the wind on what looks like little pieces of cotton. Harvesting the seed is fun, too. Just let them mature enough before 'picking' them.

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Heidi zone 4/5 Nebraska

i was also thinking milkweed on the top left ..

and the red one is a different plant ..

an actual pic of a leaf... rather than a shoot .. would probably help ...

ken

"White and milky" is usually a sign of latex sap, which can cause dermatitis, a rash similar to poison ivy. It's miserable to accidentally find out you're one of the unlucky few susceptible to getting a rash from it. This does not make a plant unwelcome in a garden, just take precautions to avoid contacting the sap when exposed by cutting the plant.

Excellent info, Xian! There are other species of milkweed, with variously colored flowers, but they all have latex sap and Asclepias is the only genus monarch butterfly caterpillars can eat.

The red shoot could be japanese knotweed ( or Reynoutria japonica). An invasive plant, hard to get rid off, it spreads by rhyzomes and gets very tall

If you work in that garden, make sure to put any pieces of the knotweed into the rubbish bin, never onto the compost heap