Welcome to gardening with the grandkids! The ratio of compost to potting soil depends on the quality of the compost and the type of potting soil. Some potting soils are very light with bits of pumice or perlite mixed in to keep the soil aerated. This means the potting soil can handle more compost up to 3/4 compost to 1/4 potting soil. If the potting soil is more dense with bits of bark and heavy when wet then you should use less compost such as a 50/50 mix to avoid a soil that is water logged. The type of compost is also important. Homemade compost may be heavy with undecomposed sticks and needles or it can be purchased in bags and then the compost is more rotted and lighter. The goal is to have soil that is rich but lightweight so it drains well. Don't worry about getting it perfect as after a year the organic matter will rot down and in future years you will have better results as long as you just add a two inch layer of compost on top of the soil each spring or fall. No need to work this compost mulch into the soil as the worms will do this for you. Keep growing, Marianne Binetti