It's actually pretty easy.
1. When you pick the kit(s) out that you want, try not to hide them, but put them in the cart/basket and try not to pay too much attention to them.
2. When you go to the check-out and if someone questions you, mention that they're for you child or grandchild, and use incorrect terminology. (For example say 'building blocks', 'Mega Blocks', 'Legos', or pretend not to be very knowledgeable about them in general.)
3. Enjoy at Home. Don't open it right away, or get too excited while you're at the store.