I love finding ways to pray with young children. This works from the age where a child can point, and works well for any age above that!
All you need to do is build two things, buildings, trains, whatever your children enjoy. Chat at your build them, and give them each a name of a blessing which is good for everyone, such as The House of Joy or The Train of Peace. Then get some play people (or animals!) and name them after people you’d like to bless, and ask your child whether [Granny] needs to go in The House of Joy or on The Train of Peace. Wherever they point or place the person, just say simple a one line prayer like, “God, as we put Granny on the Train of Peace we ask you’ll give her peace.
I’ve found this simple activity is a great way to bring prayer into their play and their lives, and it doesn’t need to last long.
You might like to let the people you’ve prayed for know what you prayed for them. I’ve often found there is a surprising correlation with what they felt they needed at that time and what we prayer for them.
You could do this as part of a creche or toddler group, and even a church service, making it possible for everyone to join in a time of prayer even if they can’t speak.