How do you ensure that junior employees contribute their ideas?

I will try my best to answer this. I do not manage people.
I am a “band EO” manager – in grades there are only two below my grade and many more above.
When I was a lower grade of “Assistant Officer” there were many opportunities to get your opinions and ideas across. Managers are usually very open to views during team meetings, or if you ask for a quiet word to have a discussion with them.
Some areas hold meetings with different grades. This gives an open and honest opportunity to ask anything you would like to know regarding their grade, the work they do, questions you have always wondered but just never had the appropriate opportunity to discuss it. Along with giving ideas on how things could be done better, charity ideas, team or department engagement ideas.
I have been involved in those meetings when I was an AO level and I found them very informative.
There are many smaller voluntary groups which you can volunteer for. These are allocated to different areas/subjects, e.g., “Engagement”, “Rewards and Recognitions”, “Digital Ambassador”, “Diversity and Inclusion” and there are many, many more. The diversity of the groups gives a wide variety of areas you could choose from to meet a variety of interests.