Show your child three objects, then hide them around one room while they wait — in easy places, partly visible at first.
Tell them how many there are and let them hunt. Keep the count out loud: 'you found two — one more to go.'
When all three are found, let them hide them for you. Watching where they choose tells you a lot about what they think you can see.
Searching for a known number of items requires holding the target in mind while scanning, which is sustained attention with a goal attached. Keeping the count aloud gives the child a running sense of progress that keeps the search going.