Echoing pp about having a strong routine. This makes all the difference. Both for them and you.
-Wake up, breakfast, get them ready.
-Put something on telly for them whilst you get ready.
-Get out the house in the morning, library, woodlands, park, soft play (bring snacks from home and go home for lunch), trip into town, walk to the shop to buy bits for lunch. It can be something small... if not in the mood to leave the house, I suspect you may find the day harder at home as I find most children get needy at home if they don't have enough one to one attention/something to do.
-Come home for lunch, feed them both. Little one upstairs for naps whilst 3 year old watches something.
-Make yourself lunch and try and have quality time with eldest if the moods is right. If eldest is happy don't intervene, let them get on with whatever they want (within reason of course).
-After nap, this can be your chill time with them pottering around at home and in the garden. Remove stuff you don't want touched. Invest in some water/garden toys.
This is a good moment to share a garden snack too!
-Prep dinner for them. Bath. Bedtime telly. Bed
-Husband makes dinner after kids in bed.
Being at home with them can make or break you. Having a robust routine will really help. Can't stress that enough.
Keep things simple. It's summer, I find I don't need to go out as much as mine hate the heat. But the garden is enough. I avoid big days out that are pricey. They aren't any happier for it. In the mornings I just do a small trip into town to get bits and bobs, see the ducks, library, playgroup, or shops/garden centre. Just to get out the house. Pack a small bags of nappies and snacks. I'm 90% always out by 9.30am and home for lunch and nap by 11.30am. Sometimes it's less than this.
Alternatively you may find you're the type who'd rather pack a picnic and drive 50mins to a park or something. If so, they nap on the way there. Eat, have fun. Drive home. They nap on the way back. Bit of telly whilst you make dinner. Bath. Bed.
It's very much trial and error and you can never get it right all the time. I've been SAHM for 5 years and still have shit days.