Routine for 8 month old with lively nearly 3 year old sister

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I have been managing so far with DS now 8 months to fit in naps around other stuff going on. However I feel he now needs to drop to two naps & can't work out how to time everything. His three year old sister is very energetic & needs to be tired out thoroughly each day or her sleep is terrible. We tend to go to a group in the morning & then go out to shops or the park in the afternoon. He used to nap on the way to things then post lunch then a late cat nap but now isn't sleepy until about 10:30 (when we are normally somewhere & he won't go to sleep) & then no tired immediately post lunch. Any hints on fitting it all in while still making DD as tired as possible (she doesn't nap at all).