I have been to Saas-Grund, but not to Bettmeralp, so I can't compare them.
Saas-Grund is a pleasant little village, but the real highlight in that area in terms of alpine beauty, traditional architecture and village ambience is Saas-Fee, which is on a plateau above Saas-Grund.
Saas-Fee is a car-free village though, so if you drove up there, you would have to leave your motorhome in the parking garage at the entrance to the village. Besides, the road up to Saas-Fee is steep and narrow, with hairpin bends. If you stay at Saas-Grund, you can get the PostBus up to Saas-Fee. It only takes about 10 minutes.
If your kids would like to see some marmots, you can get the gondola from Saas-Fee up to the Hannigalp, where there is a colony of marmots adjacent to the restaurant. I have attached some photos I took of the marmots and the view from the Hannigalp last year in mid-September.
Some things you can do apart from going up to Saas-Fee include taking the gondola from Saas-Grund up to Hohsaas via the mid-station of Kreuzboden, which is a great ride with wonderful views. There are restaurants at Kreuzboden and Hohsaas, and a playground at Kreuzboden.
You can also drive up to the Mattmark reservoir, which is quite picturesque. There are some hairpin bends on that road, but you can go up in the PostBus, which takes about 25 minutes. There is a restaurant overlooking the reservoir.
The other thing that comes to mind is to consider how comfortable you are in taking a motorhome along the Sasstal (Saas Valley) Road. PostBuses run along that road at half-hour intervals in both directions, and have right of way when there is not enough room for both vehicles on the road. On occasions, that may involve you having to back up. Those drivers are very skilled and experienced, and drive along the side of a mountain on a narrow, winding road at a cracking pace. You wouldn't want one behind you that was intent on keeping to the timetable! As there is only one road in and out of the valley, you may also encounter trucks.
I wrote a description of my experience travelling through the Saastal last year on the PostBus, and have copied a bit of it here:
The trip through the Saastal is superb, with best views on the left when travelling in the direction of Saas-Fee. The valley is steep and narrow, with the bus travelling high along one side of the valley, and lots of picturesque villages with traditional houses to be seen below or perched halfway up the mountains on the other side of the valley. From Visp to Saas-Grund, the bus climbs 900 metres. The winding road has many tight bends and a few avalanche galleries. On several occasions we met other buses or trucks on tight bends. You have to admire the skill of the bus drivers as they negotiate such tight bends safely. They sound the distinctive PostBus horn to warn oncoming traffic of the approach of the bus around a blind or tight corner, where the bus will need to use most of the road to get around the corner. On one occasion, an oncoming truck had to back out of an avalanche gallery on a bend to let the bus through: the PostBuses have right-of-way! During the final 10 minutes of the trip, the bus climbs another 250 metres in altitude from Saas-Grund up to Saas-Fee, making this last stretch a great way to end such a wonderful bus trip!