Hey moose....check out this chart
You'll see that there is a "booster stage engines" section on the chart. There are some booster engines B6-0, C6-0, and D12-0. These are the available size of "booster" engines that will go in the first stage of your rocket. Check the specs for your rocket in the packaging that you got. There should be some specified engines that are recommended. Also, just because your wife got a multi stage rocket, that doesn't necessarily mean that you "have" to fly it as a multi stage rocket. You can simply not use the booster part of the rocket.
Check out this rocket here:
https://jonrocket.com/store/products/hyper-bat/cat-1_39_48
On the line that states "recommended motors", you'll see: A8‑0/A8‑5, B6‑0/B6‑6, B6‑4, B6‑6, C6‑0/C6‑5.
That states that they recommend, for example, a C6-0 for the booster stage and then a C6-5 for the second stage. This means that the booster engine, (the C6-0) will blast off for 6 Newton-seconds and then will have 0 delay (from the time the boost ends to the time the ejection charge ignites the second stage). This is very important with multi stage rockets. Then, the C6-5 will be ignited and it will blast for 6-Newton seconds and then will have a 5 second delay, to allow the rocket to reach apogee and then the recovery ejection charge "pops" and your recovery system is, hopefully" deployed.
Things to remember, an A engine will have twice the power of a 1/2A engine. A B engine will have twice the power of an A engine. A C engine will have twice the power of a B engine....and so on. You can use that chart, above, as a reference for maximum thrust vs AUW of your rockets.
A suggestion for you would be to buy a "travel size" bottle of Baby Powder and keep it with your flight box. When you fold up your parachute or streamers, be sure to use an ample amount of Baby Powder on the inside and the outside of your parachute or streamers. Although your recovery wadding will be used to protect your recovery system from the heat of the ejection charge, the powder will be used to help make sure that your recovery system opens and deploys on those humid Summer days. I have always used an excess amount of power to make sure that my recovery systems deploy and open, and you get to see a bit of a "powder poof" in the sky and that just helps me know that my recovery system has opened properly. Once the rocket is on the ground, and you are ready to go again...add more powder. When you are done for the day, add more powder to your recovery system before you put it away. That will help with protection, too.
Another tip....in regard to "Recovery Wadding"....that Estes stuff is great. It works everytime and will not burn. However, it's a bit expensive. A "cheap" solution would be to use that rolled up tissue paper that you can buy for parties. Check this out:
Crepe Paper Streamers 81-foot Roll, Coral: Coral Paper and Plastic Dinnerware
That roll, for example, is 81 feet long and it costs a mere $0.89. That will last for hundreds of flights and if you do recover the tissue paper from the previous launch, it can always be used on your next launch. You can go to a Dollar Store and get this stuff for "cheap". Just....be sure to add enough tissue to protect your recovery system from the ejection charge but be sure to not use so much that everything gets jammed in your rocket tube.
Ok...I think I'm done, lol. If I can be of any further help....just ask! I'll be around.
